"All you can eat" should mean exactly that. It doesn't say "All you can eat - within reason , or how much we say you can eat". If you get a 400lb fat ass fuck that can put you out of business, then tough shit. You should stipulate a maximum amount.
Seriously, this is the land of the greediest, fattest people on the planet and you're tempting them in with the promise of as much as they can eat. You can't cry when it doesn't go how you want it to and slothful blubber-guts come waddling in and call you on it.