The point of the person you're replying to is that people have gotten bigger, which is a fact. Pointing out the comfort of levels of people in airplanes today doesn't dispute that.
It used to be more common to have a flexible combination of seats/baggage/fuel. But pilots flip out (or crash) if they can't fill the tanks, every seat and the baggage compartment and come in under gross. So the same airplanes often don't have the "bonus seats" they used to.
Numbers from Wikipedia, from the 1972 PA-32 owners handbook:
3,400 lb gross - 1,788 empty - (4 hours * 15gph * 6 lbs) / 7 passengers = 178 lbs per passenger. The average adult in the 70s was about 160. So you're not going across the country, but you could safely do a day trip with a 90 minute flight each way.
Now the average adult is 180. And they're a little taller than they were in the 70s, but not much. So every passenger has an extra 20 pounds horizontally. So in 2021, you'd be just over gross except that the people can't actually fit in the airplane.
