Lots of bottle reared lambs - and calves - get pot-bellied, yes. But the OP says hers haven't before, and for the most part, mine didn't.
I have often thought it a consequence of over-feeding, which can be as a result of reducing number of feeds too quickly so that each feed is larger, or feeding by numbers instead of by looking at the animal, or thinking it kind to let it have a bit more than it really needs.
The year I did have lambs pot-bellied, I'd had someone helping with feeding them, who wouldn't have had the experience to know when a lamb has had enough and needs to be taken off the bottle. And the one pot-bellied hand-reared calf I've had was fed by others, who always gave it more than I said it should have.