But here in the u.s. the average height stats are 5'10 for men and 5'4 for women. that's 6 inch difference. Why do men end up so much taller? what's the biological reason? differences in puberty, etc. ?
Being tall means you could see over high grass and brush better than someone short, it also means you can reach things like fruit off the ground easier. Us humans are two legged for a reason.
Taller men could see better over things, reach farther up and would have longer strides for running. So they could survive easier and do their job as men (killing things) easier. Women didn't need the extra height.
Also, generally speaking, males are usually bigger than females in a lot of species. So it makes sense.
Another thing is that while there are "short" men in our modern world, these men are a lot taller than our ancestors. 5'6" was the same as 6' in perspective at heights at one point.