It's not a good enough proxy for more people. If you compare the least and most costly items on a menu, even adjusting for size, you'll find that there's often a 2x to 4x difference between them. If I pick one over the other I will also have to pay 2x to 4x in tip. That just doesn't checkout.

Also, why do you need a proxy for the number of people when you can just count them? In Europe a service charge is relatively common and it's a per-person fixed charge.