I'm going with the taller people can generally be heavier people (Skinny lanky of course, do not fit this bill). And heavier people are generally stronger people. Evolutionarily speaking, there are no weight classes. I'd much be a heavy weight over a lightweight in a confrontation.
But just to be annoying... When you get very tall (anything over 6'4'', or around abouts) your strength can begin to suffer, unless you really, reaaaly get huge. For some perspective on the hugeness, in a strength sport like powerlifting (basically big people moving heavy weights) a guy of around 6'7'' who wanted to get into the sport was told he would have to be at least 375 pounds (170kg) if he wanted to ever be a world class lifter. When you get absolutely massive, your levers (arms, legs, hips, back, etc) become so excessively long, that you begin to gain a disadvantage. (eg, two people, equal weight, but 1 guys has arms 6inches longer than the other - Who is going to have a easier time bench-pressing?)
Though, i wouldn't say this is the reason the example of given of the little friend beating up on people. That may just be 'little-man (girl) syndrome' where they are out the prove something, or due to their size, over-commit to any actions.
meh... I dunno.
I'm talking out of my ass here.