Aj had much more muscle on him than Klitschko. Overall Klitschko has the larger frame. Broader shoulders and thicker wrists and yet Aj had almost 10 lbs on Wlad.
Wlad had stamina issues because of bad pacing , breathing pattern and panic attacks. The stamina itself was not bad. Just compare him to Corrie Sanders who was the opposite. Relaxed, good pacing and breathing but poor stamina.