That may be true (I've seen it myself), but "some" is the operative word there. Meanwhile, MOST large folks will eat more than small(er) people. Put another way, on average most large(r) folk's food bill is greater than small(er) folk's bill.

Compare, for example, a little old lady vs. a NFL lineman (to use an example of realistic extremes).

A pound is a pound. Tickets prices should be based upon weight. You all know this to be true, fair, and equitable. Weight, and it's costs, bear a massive penalty in aviation. Charging larger people more per ticket would not be discrimination, it would be reality as imposed by the laws of physics.