In fact, another thing to consider.
Considering the 3' person and the 9' person (clearly this is an extreme case but we can make it less extreme and the points are still valid, just to a lesser extent), which persons muscles do you think would have had to do more work over the years as they've grown up. I think it's safe to say that in general, taller people are heavier than smaller people and hence the taller person's muscles would have to work harder and hence become stronger.
Equivalently, if a 9' person shrunk to 6' but his muscles remained the same strength do you think he'd have an easier time moving, doing pull ups, climbing stairs, etc. I think so.
I hope I've persuaded you that taller people are stronger than smaller people, in general.