Why is my tetra reeeaaaally fat?

I have 10 neon tetras in a 20 gallon tank at the moment. One of them is really fat, and its stomach almost looks as if it has a bunch of little bumps. I know it's highly unlikely that it's carrying eggs, but it was this fat when I bought it too. I've researched tetra disease, and I don't think that's it. It has all of its color, and all the others are skinny, so I'm also pretty sure it's not overfeeding either. Can someone please tell me why it's so fat?