What's up with my neon tetra?

I have 2 (male) blonde tuxedo guppies and 3 neon tetras. I'm fairly sure that my smallest tetra is male, but the other two could be female. My biggest one (I named it Rainbow Brite) is fat. Like, unproportianatly. She's longer that the other fish, but she's still kind of... chubby? Is she pregnant? How can I tell? How can I tell wheter she's really female or not? Thanks :)