What would pilot or cabin crew do if a passenger were to complain that an overweight person sitting in an overwing exit row was a potential hazard?
Well, I flew El Al not long ago outbound TLV on a 737-800. First emergency exit row, left hand side, was occupied as follows: Mother with infant on aisle seat, two kids of roughly 6-8 years in middle and window seat. The cabin crew didn't bat an eyelid.
I probably should have said something, but it's really not my job is it?
BMI are very good at asking if you're comfortable sitting in an exit row, and will ensure you've read the instructions for sitting there. Have experienced BMI crew moving passengers away from exit rows if they were unfit to operate the door (too young, old, fat or stupid).
BMI does not charge extra for the exit row.