They are mislabeled and interbred a lot. Ideally, a Comet's tail should be at least the length of it's body, if not longer.
:) I've got a soft spot for them. I have a farm, and every spring rescue a few feeders, to live in the stock tanks and eat mosquito larvae and the bugs that fall in. In the fall, I sell them as decorative pond fish, because by then they've grown a lot!
I always try to pick out the comets, because they look so graceful as they grow.