Several things can factor in a vehicles towing capacity such as the brakes, frame, weight of vehicle, transmission size and quality of the transmission and the vehicles engine.
A bigger and heavier truck can tow more weight than a smaller lighter weight truck because it usually a stronger frame, bigger engine, bigger brakes, better transmission, better axles etc.
If you exceed the vehicles towing capacity it could cause steering problems, braking problems, engine strain and even transmission damage by towing more than the vehicle was safely meant to haul.
A larger vehicle can always tow more than a smaller vehicle because of the bigger engine, transmission, stronger truck or vehicle frame and sometimes dual wheels or axles.