I'm small(ish), but I have a pretty big appetite. Hence, pre-eating.
Reasons for pre-eating:
(1) When I think the restaurant we're going to is going to be expensive. (So that I'll eat less and hence spend less money. Mostly in Japan. ha!)
(2) When I don't think the food is going to be that good. (So I don't have to eat too much.)
(3) When I don't think the food will be enough.
(4) When I'm going to meet someone for the first time, so I don't have to concentrate too much on the food and not enough on the person. (I talk very little when I'm nervous about meeting someone new, ie. for work/interview/date.)
Does anyone else do this?