Simple, find a pic of anything, put it in a blank document and save it as every file format possible, it's properties will show a sizeable difference with some formats, especially with pictures or photos.
A JPEG will generally be smaller than some others like a WMF or a TIFF etc etc by quite a large amount.
Experiment with your Sig picture, when only given a small amount, it can sometimes be interesting getting it to fit the size and still be visible in some sort of quality.