Corns can grow at very different rates and this is especially obvious when they're small and growing fast. The observation that this difference shouldn't be noticable "until they hit puberty" (whatever that is in a Corn!) isn't one I'd agree with personally. It's certainly not what I've experienced over the years. Males and females aren't automatically different sizes, either.
Some bloodlines produce particularly large or small Corns. Some may have been slow starting eaters before they were bought. They may have been kept in different conditions. Their metabolisms may be faster or slower... There are so many factors which influence how fast Corns grow and how they put on weight.
As long as they're both eating reliably, pooping normally and you have no reason to suspect parasites or illness, then I'd say you have nothing to worry about.
