Who do bullies pick on the most little kids or big kids?

Bullies tend not to make such distinctions. They bully whoever they can get by with bullying the most in whatever specific circumstance. Some may pick on short, skinny kids more, while others will target obese teens. Their rationale may be that the little kids look like freaks, or that the obese kids chose to be fat or don't respect themselves. While different bullies have different criteria for victims, and they tend to have different superficial reasons for why they do it, they all seem to suffer a need to try to control others or to get attention or a laugh at someone else's expense.