Knee flexion is likely the primary driver, but not the only driver. Thigh torso plays into it as well, and is not generally an issue on a mountain bike. Also, some riders have expressed to me that stance (length, not width) plays a role in their perception of stability while descending. I would generally put a mountain rider on the same length as they would ride a road bike, which is generally 5 - 15mm longer than their TT bike (And still a touch shorter than stock road lengths in most cases). Out of 3000+ fits, I've done about 20 mountain bike fits, so not my specialty.