Fat Zebra Danio, Egg bound or illness?

I've had 2 healthy Zebras for over 2 years in a heavily planted 6
gallon tank (with 2 cardinal tetras and 2 rasboras.) One was always
chubby and the other slim, so I assumed I had a male and a female,
although I have never seen any evidence of spawning (except for the
first week I had them, when they were the only 2 in there I saw a
couple of zebra fry in the tank). So here it is 2 years later and
"male" now has a huge belly bigger than the "emales. He/she is acting
normal and does not have the "pine cone" scales, which I've read
indicate dropsy. Eggbound female or sick (or fat) male? Can't be a
female, can it? Could it have taken 2 years for it to become
reproductively mature and start producing eggs? So it must be a male
and he's either got some disease or is just a fat load. Perhaps a
parasite? I've read that 2 years is pretty long life for zebras, so
maybe he is not sick and is just getting a big spare tire in his old
age? Thanks.