[PK]

Actually, selling down your Move is the most appropriate thing, followed by perhaps taking a point less ST than you normally would.

Reducing your Move is the equivalent of having it reduced because of encumbrance. (And before anyone starts throwing math around to prove me wrong, note that I said 'equivalent'. I'm aware that there are differences when you add or subtract a certain amount of external encumbrance, and frankly, it doesn't matter.) If your fat would have reduced your Move by 2 points in 3e, just reduce it by 2 points manually in 4e, and take the disadvantage points for it.

And then, because in 4e ST represents "your applicable ST after factoring in your ability to carry yourself", it might make sense to drop ST by a point -- which really just means not buying it up as high as you normally would -- to reflect the effort you're having to put into just lugging yourself around. But that's a maybe; I mean, you could justify any level of ST, just by saying that if he weren't Fat, it'd be a point higher.