Plane seats are not made, neither is anything else, for body sizes 2 and 3 or more standard deviations off “normal adult size”. Unless so advertized.
If I remember correctly airline seats are based on 1950’s demographics, and are indeed too small for today’s average size passenger. That being said, most everything in aviation (getting it off the ground and keeping it there) is concerned with weight (gross amount and distribution in the aircraft). Huge individuals should expect to pay more for their weight and the space they occupy. Why should the person who has paid for a full seat be denied less than that because the behemoth next to them expects everyone else to pay for their excess?