If there are proper warning signs at the beginning of the queue that not only state the sphiel about pregnancy, back/neck injuries... etc, but clearly outline the suggested maximum weight limit, there should be no problem or complaints from the 'large' people if they waited over an hour for the ride only to be told they are too large to ride.
I have nothing prejudice against them... but if, and let me stress if... if there are proper warning signs, and the common sense ticker in their head is working, it should tell you that maybe this coaster isn't met to your specifications like a lot of other things in life that are, essentially, made for the "average" person (ie: movie theater seats, bucket seats in sports cars, elevators, bicycle seats).
If there is no sign... well... tough luck on both parties, because I don't wanna be the ride-op in that situation.