That’s a good question and one we get fairly often. The reason for the height and weight restrictions is the FAA’s concern that with a heavier traveler strapped to a seat back the seat would tilt forward more quickly in a short stop, causing the person sitting behind the seat not to be able to brace himself.
Airplane seat backs are designed to “give” when pressure is applied with bracing. This is important for the safety of the person sitting behind you. The FAA placed the height and weight restrictions to make sure that all passengers would be covered in the event of an emergency.