Large != Fat, at least not always
Not all large passengers are fat.
I might weigh more than I would like at 13st but it doesn't in anyway impact how wide I am.
In the case of many blokes, the widest point is across the shoulders and not across the backside.
The cattle class seats on most airlines are now pretty marginal for seating 6 foot blokes. They have completely failed to take note of the increase in average sizes of people over the last few decades. On a good day I'm 6', when I was twenty that counted as tall, but certainly not unusual. These days 6' is pretty average, one of my 12 year old kids younger friends is already taller than me, OK not yet wider across the shoulders but give him a couple of years and he will be.
On BA long distance flights they normally have 4 classes, my customers will only pay cattle class, most are like that. I can often afford to pay the uplift to cattle+ class where there is plenty of space for me. The difference between cattle & business class is often more than a factor of 5, well I ain't that rich it would normally consume all or more of my fees.
I used to try and get exit row seats to get more legroom, but these days the exit row is normally full of larger than average blokes, most of whom aren't large due to eating too much. So putting three of us into a row of those 3 seats is a tight fit.
Airlines should be forced to have a minimum seat size which at least bares some resemblance of the expected size of healthy passengers.
On the other hand, I've refused to have my arm rest raised before now.