I'm assuming that being short and heavy, that a lot of that weight is protruding out the front? If so, then there could also be the simple problem of the seatbelt not fitting.
Heaviest passenger I've flown, was 21st and about 5 foot 5. Put him in the back of the banger on the middle seat and used the lapbelt extender to connect the belts either side of him. He wrote a letter of complaint because he wanted to sit in the front!