The size is not really how big the sig LOOKS its how big it actually IS. You can right click on an image to see the file size.

A person could have a pic that looks very large and actually be much smaller in file size than another persons tiny animation.

You also have to remember that the file sizes add up. You may have 3 images that are all under the 25k limit each, but those 3 added together may be twice the size of the limit.