It is not only overweight people who may or may not smell. I have sat next too many a larger person with no problem at all. I have also sat next to thin people who last saw a shower in the 60’s.
Long (and short) greasy haired people (of both sexes) who do not apply deodorant, and even, mainly women here, who have the habit of fluffing up their long hair, letting hair and other “dust” fly out over me or worse, my drink or meal.
So let’s not discriminate on the basis of size. It would just be nicer if people recognize that traveling in small confines means a little more consideration towards their neighbours.