Big as in muscular or tall? Or big as in 40+ inch waist?
My trouble fitting in seats is at the shoulders and arms primarily and leg room secondarily. I do ok in aisle seat because I can have my shoulder out in aisle a bit. I have no problem with my waist fitting.
So essentially I am 35 pounds heavier than my ideal weight, but it would make no difference for how I fit in airline seats one way or the other.
For me - 38” waste (36 in high school) and 52” chest. My shoulders are broad.
When I have the middle seat I always end up with knots in my shoulders from hunching forward to give everyone room.
And I’m a 90K+ flyer.