I just checked one of the big chains. A single slice of plain cheese pizza is 295 calories. So an 8 slice "large" pizza would be 2,360 calories. Don't get me wrong, I've done it before, but that's a lot of food for an average adult.
Assuming that's accurate (I feel they may be overshooting here, but I guess it depends on the chain), that's little more than daily requirement for a single adult.
It matches my experience - I can keep myself fully fed for a whole day on a single large pizza. It also reinforces my point - such a pizza might suffice as a dinner for two, but will hardly satisfy more people.
