i have only sat next to a man mountain once, about 20 years ago and thankfully only frm bkk to india. even so it isn't an experience i want to repeat, particularly as he was suffering from BO and what had to be terminal halitosis! The stewardess kindly found another seat foir the third person in our row, a woman, so Mr Porky had the window and middle seats and I had the aisle. fortunately he didnt need to get out for any reason and it was only a flight of a few hours anyway.
But should these people should be charged for taking up two seats? IMO I think they should, after all the airline loses the revenue of the second seat. As an aside I can't recall anywhere on the booking form that asks for your weight.