Image compression vs image resize

What compression does

Image compression reduces file size while keeping the image width and height the same. For JPG and WebP, this usually means lowering quality slightly so the file becomes lighter. It is useful when a photo already looks the right size on screen but exceeds a file size limit such as 5 MB or 10 MB.

What resizing does

Resizing changes the number of pixels. A 4000 x 3000 photo contains twelve million pixels. If you resize it to 1600 x 1200, the visual dimensions become smaller and the file often becomes much lighter too. Resizing does not have to crop the image; it can keep the full picture while changing width and height.

Which one should you use?

• Use compression when the image dimensions are already correct but the file size is too high.
• Use resize when the image is too wide or too tall for the place where it will be uploaded.
• Use both when a camera photo is very large and also needs a small file size.
• Avoid unnecessary re-saving because each lossy export can reduce visual quality.

Practical examples

For a website thumbnail, resize first because visitors do not need the full camera resolution. For a document upload that accepts the current dimensions but has a strict file size limit, compress first. For a batch of phone photos, resize to a reasonable maximum width and then compress to a balanced quality setting.