Resize image changes the pixel size. A picture of 1920 × 1080 can become 1280 × 720. A square of 1000 × 1000 can become 500 × 500. This is not the same job as compressing a file: a smaller pixel grid usually makes a smaller file, but the point is the dimensions.
Keep the aspect ratio when the picture should not stretch. Turn it off only when you want the exact width and height you typed, even if the picture is squashed. Compress image if the dimensions are already right and only the file size should drop. All tools