The file size checks for how many pixels the image contains in bytes. For example, an image of 2,340,000 pixels has a file size of 2,340,000 bytes or about 2.34 MB (this is because there are 1024 bytes in a kilobyte and 1024 kilobytes in an MB).
The recommended file size of images is less than 100 kilobytes for a fairly small image, and within 200 kilobytes for banners and large images.