Weight is largely irrelevant. The difference between a airliner full of 45Kg people versus 90Kg people constitutes about 5% of the total plane weight. Some fuel savings but would not be worth the aggravation, client humiliation, lawsuits etc.

What it boils down to is volume and time. A passenger takes up a seat regardless of how large he is (...) so he's charged for a seat. Luggage changes that equation far more than passenger body weight.