

No it isn’t. Not in large airplanes. The typical takeoff weight of an A320 is in the neighborhood of 50-70 tons. The pilots do not give a shit if a few passengers weigh a couple 100kg more than the average, and that’s a narrow body. A couple PAX being grosly obese on a widebody, with typical takeoff weights in excess of 100 tons, is even more negligible.
Fuel calculations and weight&balance is calculated based on assumed average weights for men, women and children, generally something in the neighborhood of 85kg for men, 75 for women and 30-40 for children (includes assumed average hand luggage weight as well)









There’s already a solution for that. It’s called premium economy/business class.
And if you’re gonna say “but that’s so much more expensive”, well guess what’s going to happen if you reduce how many pax can fit on an aircraft.