Compression changes encoding
Compression changes how image data is stored. For JPG and WebP, lower quality settings can reduce file size by accepting some visual loss.
PNG compression behaves differently and may not shrink every file much, especially if it is already optimized.
Resizing changes dimensions
Resizing changes width and height. If an image is 4000 pixels wide but only needs to display at 1200 pixels, resizing can remove a lot of unnecessary data.
Keep aspect ratio locked when you want to avoid stretching.
Use both when needed
A good workflow is to resize first when dimensions are too large, then compress if the file is still bigger than needed.
Related ConvertZiva tools
FAQ
Which reduces file size more?
It depends. Resizing can help most when dimensions are excessive; compression helps when encoding quality can be adjusted.
Can compression damage quality?
Lossy compression can introduce visible artifacts at aggressive settings.