I recall reading that female corys of any color would be rounder and fatter than the smaller males. It is possible you have a female and the others are males.
On a side note, I think it's hilarious that I have three coreys and you and I have the exact same mix. Small world, eh?

My corys are all quite old, my green is 14 if you can believe it. They are all about 2 inches or so with the albino being the largest. I think that one is about 2.5 and probably a female.