To use BMI as criteria is absolute tosh. I have a BMI of 28 and therefore overweight. I could easily move up to obese (lardass if you prefer) and still not be encroaching into any adjoining seat.
This is because, apart from being a lardass, I am also a shortass at 1.57m who could never become wide enough to be in contact with both armrests let alone encroach into adjoining space. But because I am overweight at 69Kgs the fanatics posting here propose that I should pay extra whilst being well below the weight of most of said posters.
Perhaps I should be entitled to a discount as I don't use all the available legroom.