Images get downsized at client side when the maximum dimensions for images are set, but the maximum filesize (applies to all attachments) gets checked to the original size of the images. Because the images get downsized, the original size is less important to check on. For example you want to allow non-image attachments up to 1M but images to 10M (they get downsized at the client side) Why 10M? Many people try to upload directly from their camara.