Here's a major problem with this: fighters only need to be in shape for their fights. If they choose to eat poutine every day in between training camps, that's their choice and they should be free to do it.
The only weight that matters is their ideal fight weight, and weighing them when they are not in fight prep mode throws off the stats.
Fighters are independent contractors. If the UFC wants to control how they behave in between fights, they need to hire them as employees and pay them year-round.
Like the major North American sports, they'd have to codify that behaviour in a CBA, otherwise courts would block the UFC from denying them employment for personal lifestyle choices (i.e. getting fat in between fights).