How Do Image Compression and Format Conversion Work?

When you work with digital images, you inevitably come across questions about file size, quality, and which format to use. Why does a photo take up so much space? Why does an image sometimes get bigger after compression? When does it make sense to convert a file to another format, and what should you expect from that process? Let’s break it all down.

What Affects Image File Size

Digital images consist of many colored pixels. The more pixels there are, the higher the detail and the larger the file. However, file size is determined not just by the number of pixels, but also by how the image data is stored. This depends on the file format and the type of compression used, since different formats and compression methods store image information in different ways.

Image File Formats and Their Differences

There are several widely used image formats, and each one is designed with different goals in mind. The way a format stores image data affects not only the file size, but also the quality, transparency support, and compatibility with various applications and devices.

JPEG is the most common format for photographs. It uses lossy compression, which means some image data is permanently removed to reduce file size. JPEG files are usually small and load quickly, which is great for sharing photos online or by email. However, JPEG doesn’t support transparency, and saving the same image multiple times can noticeably reduce quality.

PNG is often chosen for graphics, screenshots, and images that require transparency. PNG uses lossless compression, so it preserves all image details, even after repeated saving, making it a suitable choice for logos, icons, and images with text or sharp lines. PNG files are typically larger than JPEGs, especially for detailed or photographic images, but they maintain perfect quality.

WEBP is a newer format developed for the web. It can use either lossy or lossless compression and supports transparency. WEBP often produces smaller files than PNG or JPEG at similar quality levels. Most modern browsers and many apps support WEBP, but some older software may not.

GIF is mainly used for simple animations. It’s limited to 256 colors and supports only basic transparency (pixels are either fully transparent or fully opaque). Because of these limitations, GIF isn’t suitable for most photos or detailed images, but it remains popular for short, looping animations.

TIFF is used in professional environments, such as publishing and scanning. TIFF files can be lossless or uncompressed, and they’re often very large. This format isn’t practical for web use or everyday sharing, but it’s valued for archiving and high-quality image storage.

HEIC/HEIF are modern formats used by many smartphones, especially Apple devices. They offer efficient compression and can store multiple images in a single file (for example, live photos). The downside is that compatibility with non-Apple devices and older software can be limited for these formats.

How Compression Works

Compression is the process that makes images smaller, but not all compression methods are the same. The technique used (lossless or lossy) directly affects the final file size and image quality.

Lossless compression reduces file size without removing any information from the image. When you open a losslessly compressed file, you see exactly the same image as before compression — no details are lost. This method is used by formats like PNG and some settings of TIFF and WEBP. Lossless compression is needed when you want to preserve every pixel, such as with graphics, logos, screenshots, or images you plan to edit multiple times.

Lossy compression achieves much greater reductions in file size by permanently removing some image data. The idea is to discard details that are less noticeable to the human eye, especially in photographs. JPEG and the lossy mode of WEBP use this approach. Lossy compression is excellent for photos, where small losses are usually invisible, but it’s not suitable for images with text, sharp lines, or graphics, because it can introduce visible artifacts like blurring or blockiness. Also, if you repeatedly save an image in a lossy format, the quality will degrade with each save.

What Happens During Format Conversion

Converting an image from one format to another isn’t just a matter of changing the file extension. Each format stores image data differently, and the conversion process can have a noticeable impact on file size, image quality, and supported features.

When you convert an image, the software reads the data from the original file and writes it into the new format according to that format’s rules. If you’re moving between formats with similar capabilities (like PNG to lossless WEBP) there’s usually little or no visible difference. But when converting between formats that use different compression methods or support different features, the results can be dramatic.

For example, converting a PNG (which is lossless and supports transparency) to JPEG (which is lossy and doesn’t support transparency) will remove any transparent areas and may introduce compression artifacts. The resulting file will be smaller, but you’ll lose the ability to have a transparent background, and sharp edges or text may become slightly blurred.

On the other hand, converting a JPEG to PNG doesn’t restore any quality that was lost during JPEG compression. The PNG will simply store all remaining details, including any blurring or artifacts introduced by JPEG, in a larger file. This is why converting JPEG to PNG is never a good idea unless you specifically need to add transparency and are prepared to edit the image manually. Otherwise, you just end up with a bigger file that looks the same or even worse.

It’s important to understand that some conversions can only go one way. Once detail is lost in a lossy format, it cannot be recovered by switching to a lossless format. Similarly, converting between formats with different color or transparency support can lead to information being discarded or changed.

Why Images Can Get Bigger After Compression or Conversion

It seems logical that compressing or converting an image should always make the file smaller. In practice, this isn’t always the case. Sometimes, after compression or format conversion, the resulting file is actually larger than the original. It can be confusing, but there are clear reasons why it happens.

One common situation is converting from a lossy format (like JPEG or lossy WEBP) to a lossless format (like PNG or lossless WEBP). Lossy formats remove a lot of data to keep file sizes small. When you save such an image as a lossless format, that new file stores every pixel exactly as it appears, including all the imperfections and artifacts left by the previous compression. Since lossless formats don’t throw anything away, the file can become much larger, even though the visual quality doesn’t improve.

Another reason is that some images are already highly compressed. If you try to compress them again, especially with lossless settings, the software can’t remove much more data. In some cases, the new file will have additional metadata or a less efficient structure, which can actually increase the file size instead of reducing it. Similarly, downscaling or editing can introduce more complex patterns or noise that are harder for the compression algorithm to reduce efficiently.

This is not a bug or a failure of the software; it’s a normal result of how different formats and compression methods work. If your goal is to reduce file size, always consider both the original format and the target format, as well as the compression settings you choose.

How to Choose the Right Format

Now that you know the main differences between image formats and compression methods, it’s easier to decide what to use in specific situations.

If you’re preparing photos for sharing on social media or sending by email, JPEG is almost always the best choice. It keeps file sizes small and is universally supported. Just avoid repeatedly editing and saving the same JPEG, as this gradually reduces quality.

For website graphics, interface icons, or images that need a transparent background, PNG is usually the right tool. It keeps sharp edges and transparency intact, which is important for logos, buttons, and screenshots.

If your goal is to get the smallest possible file size for the web and you know your audience is using modern browsers, WEBP is worth considering. It can deliver smaller files at the same visual quality as JPEG or PNG, and supports both lossy and lossless compression as well as transparency. Make sure to check that your platform supports WEBP before making it your main format.

When you need to keep every detail for future editing or high-quality printing, use lossless formats like PNG or TIFF. These are also useful for archiving originals, even though the files are larger.

Animated images are a special case. GIF is still widely used for simple animations, but for better quality and smaller files, animated WEBP or even video formats (like MP4) are often preferable if supported.

Whenever you convert images, remember: converting from a lossy format (like JPEG) to a lossless one (like PNG) doesn’t restore lost quality and usually increases file size.

If you’re unsure, save a copy and try different formats and settings. Compare the results visually and check the file sizes — it’s the best way to find what works for your specific case.