To follow up what I said about mentally dividing the amount of food by the amount of people, so I know what my share is--I know some people, generally reasonable and polite, who don't do that, because when they see "a lot" of food, they assume that whoever provided it knew what they were doing and thus there's enough for everyone to have a "normal" amount. I actually think that's quite reasonable and not at all rude, it's just that unfortunately, those assumptions are not always true.
For example, I was tasked with ordering pizza for the office, which I had little experience with. The boss was paying for it out of her own pocket and had an unrealistic idea of what pizza cost these days, so I was very conscious of not appearing, to her, to be frivolously spending her money. I figured 2 slices/person, because that's what I eat, and I think it was 7 people so I ordered 2 pizzas (16 slices, so a 2-slice margin of error). At the last minute my boss added several extra people to the lunch
and of course I was nervous about what I had ordered so I was watching everyone closely. I was pretty surprised to see so many people take 3 slices--couldn't they see there wasn't enough even at 2 slices/person?
And then when I came here to post about it I realized that for a lot of people, 3 slices is normal, so I was wrong in that assumption. And I didn't
know what I was doing, and also was constrained by things outside of my control, so there really wasn't
enough for everyone to take their "normal" amount. Ideally there should have been, but from my perspective it would have been better if people did a quick count, realized something was wrong, and tried to be part of the solution (taking only 1 slice at a time) instead of part of the problem. But, I can see how that would be well outside the normal thought process for a lot of people. Obviously in this case I was highly invested in seeing how much people ate.