The three main things that most affect airline comfort for a big passenger are seat belt extenders, seat space and restroom facilities.
How to Get More Airline Comfort for a Big Passenger
Seat Belt Extenders
If the seat belt on an airplane doesn’t fit you, you need to use a seat belt extender. All airplanes carry them — flight attendants use them to demonstrate how to fasten your seat belt. Airlines win points from their passengers for being discreet and polite with seat belt extenders. If you get a pre-assigned seat, you may be able to ask the airline or your travel agent to put an extender on the belt for that seat in advance. Otherwise, you can ask the flight attendant for one.