Resize vs Compress Images: What Is the Difference?

Two different things

Resizing means changing the number of pixels in an image: its width and height. A photo of 4000 by 3000 pixels, resized to 2000 by 1500, contains a quarter of the pixels. Compressing means changing how the pixels are stored in the file, for example by using a lower JPEG quality or a more efficient format, without necessarily changing the dimensions.

Both reduce the file size, but they work in different ways and have different side effects. Understanding the difference helps you choose the right tool for a problem, and it often shows that combining them gives the best result.

How resizing saves space

File size depends on the amount of detail to store, and the number of pixels is a main factor. Halving the width and the height reduces the pixel count to 25 percent, and the file shrinks by a similar proportion, often a little less. Going from 4000 to 1600 pixels wide leaves 16 percent of the pixels. That is why resizing is the strongest single step for pictures that are bigger than they need to be.

The side effect is a loss of resolution. A smaller image cannot be enlarged again without becoming soft, because the missing pixels are gone. If you may need a big version later, keep the original.

How compression saves space

Compression keeps the dimensions and reduces the data per pixel. At a quality setting between 70 and 85, a JPEG is typically much smaller than at 100, and the picture looks nearly the same. The side effect is the appearance of compression artifacts if you go too far: blocks, halos and banding.

Compression also includes the choice of format. Converting a photograph from PNG to JPEG or WebP is a compression change that can reduce the file by a large factor without changing a single dimension.

When to use which

If the image is bigger than the size at which it will be displayed, resize first. There is no benefit to sending 4000 pixels to a screen that shows 800. If the image already has a suitable size but the file is still big, compress by lowering the quality or changing the format. If you need the smallest possible file, do both: resize to the display size, then compress.

The order matters in tools that process in separate steps: resize first, then compress once. If you compress, resize and compress again, you add an unnecessary round of loss. The compressor on this page does both in a single pass from the original, so there is only one lossy step.

Retina and high-density screens

On high-density displays, an image may need more pixels than its size on the page. An image shown 600 CSS pixels wide looks sharper on a phone with a 2x display if the file is 1200 pixels wide. This does not mean that you should always supply double size; for photos that are mostly content, 1.5x to 2x of the display size is usually enough.

The srcset attribute lets the browser pick from several versions of one image. Create each version with the maximum size option, and let the browser choose the right one for the screen.

Quality at the same file size

At a given file size, a smaller image with high quality often looks better than a large image with low quality. Think of a 4000-pixel photo compressed to 100 KB: it shows blocky artifacts because the encoder has too little data for so many pixels. The same photo at 1600 pixels can use that data budget for finer detail per pixel and look cleaner.

This is the reason why shrinking the dimensions is usually preferable to pushing the quality very low. Try both on a photo, compare the results at the display size, and you will see that the resized one is more pleasant at an equal file size.

Practical settings

For a website column, use a maximum size of 1280 to 1600 pixels and a quality of 75 to 80. For a full-width banner, use 1920 pixels. For email, use 1600 pixels and 70 to 80. For small thumbnails in a list, use 640 pixels and 70. These are starting points; check the result for your own pictures.

Both options are available at once in the compressor, and a change in either one reprocesses the list immediately. Compare the sizes and the thumbnails, and pick the combination that gives the smallest file that you are happy with.

Common mistakes

The most common mistake is to resize an image twice, once in a phone app and once again in an online tool, because each step adds a round of loss. Another is to enlarge a small image to meet a minimum size: the picture gets no sharper, and the file gets bigger. A third is to compress a screenshot with text to a very low quality, which makes the letters fuzzy.

If a result looks worse than expected, go back to the original and try again with a higher quality or a larger size, instead of compressing the compressed copy.