I think the reason why you see larger sigs is maybe because they had their sigs loaded up on the site before the size restriction. not really sure though.
right now there is a max limit on the pixel size 500x100. which is more around the size of my sig. with the proportions of your picture, when its up loaded, im guessing the website is trying to fit it within the 500x100 size and its hitting the 100 pixels height 1st. hopefully that makes sense. Try remaking the sig to the 500x100 size and see if it works.