Well, it's not how much you eat but WHAT you eat.
Buffets usually, at least here in Finland, include great salads too. And you don't have to take the greasiest meat of the buffet... You can always take some greenies, veggies and fruits a bit more.
And of course, your digestive system (I don't know if this is a proper expression but hope so) plays a big part too.
And yes, in Finland it seems that truckers overweight probem needs some kind of solving too. Though, conditions (HOS, truckstops, trucks, schedules etc.) differ hugely from those in there.
-Janne-