Large characters with a grinding bike (that is, a bike that turns very sharply) have the advantage most of the time. Excellent top speed, turning, weight (hard to push around), etc.
Light characters have the advantage if they're getting hit hard, and their good acceleration will save them.
Medium characters tend to be good for beginners or if you just don't feel like picking sides or taking a risk (Is this gonna be a smooth match or an item-heavy match?).