Yup, I agree. There is such a thing as live weight and dead weight when it comes to maneuvering and getting through the technical stuff. The live weight (the rider) has to move the dead weight. The heavier the dead weight the harder it is to control with the live weight. A heavier guy will likely be able to throw the heavy bike around more easily than a light guy to get around or over roots and rocks and tight turns at speed.

When it comes to climbing, the heavier guy pays two penalties: (1) just moving more mass against the acceleration of gravity (force = mass x acceleration) and (2) he's taxing his cardio vascular system (includes his aerobic capacity) more if he's heavier due to excess fat. Bigger low-fat guys have bigger lungs and cardio systems, but they still suffer a disadvantage due to the restraining force of gravity on climbs.

Al