opinions on enclosure size?

[poomwah]

I'm a bit stumped on this one.
I have my 19 month old eastern rat, George in a 55 gallon tank, he's been in there since I got him, and I have NO problems with him at all, he's social, has a great appetite, fed once a week, 261 grams, no problems at all.
I've got a 17 month old yellow rat that doesn't want to eat, she's 29 grams, I've been told that she's not eating because her enclosure is too big, that should should be kept in a 5 gallon to 10 gallon at the maximum for 3 to 6 months, then should be gradually moved to larger enclosures with a week or 2 off feed for acclimation each time I move them up in enclosure size.
This seems really crazy to me, but that doesn't mean its wrong.
I've been told the same thing about my bull snake Norma Jean, that her enclosure is way too big and she will never feel safe. She's 6 months old and in a 55 gallon. I called her breeder about it and he said that although a 55 was way bigger than what she needed that there was nothing wrong with using it.
I think Ziggy has his young bull in a 40 if I remember right.
So, if a snake has plenty of secure hides, how much difference does it make in how big the enclosure is,
what do you guys think?