I was the skinny (very high metabolism, don't gain a lot of weight/fat or muscle) 4'9'' male (now 6'0'', though still very high metabolism) in my class that had lived a rather sheltered life with a fundamentalist Christian parent whom enforced the "turn the other cheek" sentiment on me when it came to teasing and bullying around me to a rather literal extent. I didn't have much confidence because of such, so I was always an easy target in school. I had friends whom would stick up for me in the worst of times, but when I was alone, I didn't really have any fighting capabilities to speak of.. besides running about like a madman, that is. I got both the physical intimidation and the verbal intimidation, in the end.
So pretty much anything the opposite of such a scenario tends to a reason why an individual would not get bullied. Though the bottom line tends to be, without a close knit group of friends on a regular basis and a strong backbone.. shit goes down.