An incredibly open-ended question. Depending on the situation, it's going to depend whether big or smaller has an advantage.

Let me just throw out the first factor that popped into my head--caloric requirements.

A bigger, more muscular person with all that metabolically active muscle is going to need more calories a day. That's something that needs to be met day in and day out, and if food is scarce or requires a lot of energy to obtain, then that's an advantage for the smaller person over the longer run.

Well, if you're "big" because you're fat, then that's likely an advantage when food is scarce, unless that makes you an easier meal for something even bigger!

As a broad generality, probably many of the situations where bulk/size is an advantage can be done pretty well by the smaller person with some brainpower and/or tools of some sort.