There are some reports along these lines. Some studies show that obesity has a negative effect on knee motion. Doctors think the force of the extra body weight causes soft tissues around the joint to get pinched between the femur (thigh bone) and the tibia (lower leg bone). The result is restricted range of motion, especially into flexion.
Other studies don't agree. In a recent study at the London Health Science Centre, body mass index (BMI) wasn't linked to postoperative range of motion.
It's possible that today's improved implant technology has done away with problems related to body weight and alignment.