DS8 is in the 97th percentile for height and weight, and has always been the biggest kid in his classes. He loves to make friends with kids both older and younger than him.
He was being bullied by a little boy his age who did not come up to his shoulder last year. So...it's more about your own confidence, how you carry yourself, and what you will tolerate from other people than it is about your size or your age, IMNSHO.
There was a major story last year in which a pro football player was the victim of bullying. He is over 300 pounds and pushes other beefy people around for a living. So you're right, it's not just about size.
In the case of the sports story, the main perpetrator was another offensive lineman himself, who was joined in his behavior by other teammates, and who enjoyed special status as a member of the team leadership council, so there still remained a significant difference of power between the victim and the perps.
Bullying is often encouraged by a perceived difference in power between victim and perp, and being larger means there's a physical difference of power, so size still matters.
