What Does Keep Aspect Ratio Mean?
Keeping the aspect ratio means preserving the original relationship between width and height while resizing. It prevents the image from looking stretched or squashed.
What to do
If an image is 1600 × 1000, its ratio is 1.6. Changing the width while keeping that ratio automatically determines a proportional height. The simplest next step is to Resize Image. If the issue is actually about image size checker, see Image Size Checker as well.
Why this happens
Image problems often come from three separate variables: pixel dimensions, encoded file size and file format. Changing one does not automatically solve the others. A very large photo may need resizing even when its format is already JPG; a transparent graphic may need PNG or WebP even when the file size is acceptable; and an upload limit may be measured in megabytes rather than pixels.
Before editing, check the requirements of the place where the image will be used. That gives you a concrete target and reduces unnecessary quality loss. When possible, keep the original file and create a delivery copy for the specific destination.
Common questions
When should I not keep the ratio?
Only when you deliberately want a different shape and accept distortion, or when another workflow handles cropping separately.
Does aspect ratio change file size?
The ratio itself does not determine file size. Pixel count, format and compression do.
What if the destination has a fixed rectangle?
Consider resizing proportionally and then cropping to the exact rectangle instead of stretching the image.