Re: might be getting a new girl
Well I don't think the "grow-to-fit" thing is impossible. Size is genetically controlled but that is maximum size. Outside factors can, of course, stunt the ideal growth rate of an animal. The not so obvious thing is what might do that - especially considering that the severly confined spaces seen in captivity aren't seen in nature and, therefore, are unlikely to have genetic code dedicated to living in them.
For snakes, the only thing I can think of is activity level. It would be possible for lowered activity level to lead to stunted growth. In fact, this is true in humans to some extent. However, I would not expect this to be a subtle effect that could lead to "grow-to-fit".
So, in the end, I would have to say that grow-to-fit is probably hogwash, at least for snakes. I've been told as much by my local exotics purveyor who said "A small cage won't give you a small snake, it will just give you a grumpy snake."