It's probably due to about 1,000 Calories per day too much. It makes us a couple of inches taller than our immediate ancestors, true. But it also makes us 40-50 lbs heavier, and it ain't all muscle.
Modern boys probably have a layer of subcutaneous fat that didn't used to be there, no hard edges. Yesterday's 5'9" 150lbs. is now 6', 210. and in all too many younger fellows,a big-butted 220-230! (Not counting the weight of the tattoos.)
Civil War: Average Union Recruit: 5'8" 140lbs!
BTW, extra fat means earlier puberty!
Yes. Girls today are heavier, and are going through puberty at a younger age.