[karomata]

I think that a large robot would be better. I don't think that a small robot would play very good defense. I also agree that smaller doesn't mean lighter.
I was thinking that a good idea for defense would to spray one side of a square robot jet black so that drivers couldn't see through it. Then just move back and forth in front of the opposing alliances drivers station as the drivers try to aim to score.

the opposing drivers wouldn't like your strategy very much, but it sure would be effective!