I had the joys of that on my last return from the US, LHS.
To be fair to him, although he needed a seatbelt extender he wasn't a problem at all as far as "overspill" goes.
Unlike the mammoth I had the misfortune of being stuck beside on an overnight Amtrak train once, who took up a third of my seat too. One decided crashing out in the observation car was a better idea then (train staff asked why I was sleeping there so I told them to go and look at where I was supposed to be sitting. They came back with a blanket for me.)