As a matter of interest, the FAA just raised the basic adult weight from 170lbs to (I think) 180lbs.
Balance is also an issue, though I assume the pax load can be evenly distributed by either an algorithm in the program that assigns seating or having full loads (right) or by the natural human tendancy to sit apart from other people.
Maybe in a larger a/c, the location of the weight of the pax wouldn't be an issue, even if it was half full and they all sat at the back.