Of course 'bodybuilding' doesn't improve performance. Bodybuilding develops superficial musculature in the pursuit of aesthetics.

Weight lifting doesn't just equate to bodybuilding... Just as Juan Manuel Marquez

Purposeful weight training should lead to myofibular hypertrophy (as opposed sarcoplasmic attained from bodybuilding), mobility and mechanical efficiancy of the skeletal muscle system and most importantly for somebody looking to improve performance withouting gaining weight - development of the nervous system, leading to greater output from the muscles.

A heavier athlete with a better developed nervous and musculature system will always hit harder than somebody smaller, assuming technique and proportions are equal.

Clearly a well developed athlete with sound technique is going to hit harder than a pumped up bodybuilder. Why wouldn't he?