Quote "First question through my mind - where did they put this heavyweight?"
There's only one place; the seat(s) that are the furthest way from any emergency exit.
Grossly fat people have rights, but they do not included blocking the way to or through an emeregency exit, any more than very fat people have a right to overflow into the space allocated to adjoining seats. They must be at the back of the queue to leave.
Neither have I any desire to be tied to the same seat row as a grossly overweight person, if and when subjected to the maximum forces the seat mountings are designed for with a normal load, as in a survivable ground impact (Kegsworth, for example) or a ditching.
If that offends grossly fat people, tough. Don't fly, unless you are prepared to accept the limitations of your self-imposed condition. And, please, spare me the delusional rubbish about how you can't help it. The only source of fat is what you put down your throat.
I never accept a seat next to a grossly fat person. I don't shout the odds, just make a polite, very firm request out of the fatty's earshot. It works, but if it didn't I would disembark and sue under contract, tort and human rights.