I can only speak to CA, but I was once a passenger in my very large friend's car and she was pulled over for not wearing a seatbelt. She literally just said to the cop "I'm too fat, it doesn't fit * insert sooky face *" And the guy was like, eh, drive on but you either need a doctor note or to buy an extender. And then sent us on our merry way - I think it helps that I had mine on.
Anyway she was mortified, obviously, but straight away slipped her seatbelt on. It fucking fit! Uncomfortably though. And I was like 'GORL?!' And she shrugged it off. Apparently it's happened a bunch of times and they don't give a shit. They clock the fatness and are like 'yeah, ok, whatever.'