All they need to do is regulate the width of all the entrance doors to the restaurants and narrow them down to the desired size and that would do the job.
They could even put up a sign like "You must be at least this skinny to enter".
But I would think these "heavy" eaters would be among a restaurant's best customers. I would be drawn to a restaurant that advertised "All the fat people eat here"....and there's something about a skinny chef that makes me suspicious.
They are the best customers of the "all you can eat" buffets.