Samoa Air charges passengers by weight (truly just self-loading freight from their perspective).
Samoa Air - How does 'Pay by weight' work?
They do offer XL seating for the extra-hefty individuals.
Elsewhere - just assuming an average weight makes for faster loading. The assumed average does get reconsidered from time to time, as sociology dictates.
FAA average used to be 170 lbs (77.3 kg) per adult - raised in 2004 to 190 lbs for adults (200 for men, 179 for women) after an accident due (in part) to an overweight condition caused by the inaccuracy of the old FAA figure (Air Midwest 5481). The estimates also vary with the season (they assume 5 extra lbs (2 kgs) of additional clothing weight per pax in cold months).