I doubt that a larger school has more bullying per student. In large schools the students tend to subdivide into non-intersecting groups (e.g. nerds, jocks, rednecks, punks, etc). I went to a medium/largish high school and there were kids in my graduating class that I had never met, did not know their name, etc. Kids in these differnt groups largely left each other alone, because they did not compete for the any of the same things. Fights were usually between members of the same group.