Perhaps the pieces of charcoal were actually too big. I imagine most of the heat reaching the steak is being radiated from the coals closest to the steak (i.e., those that are sitting directly on the grate in the chimney starter). If that's the case, you'd want to break up those large lumps and pack the chimney starter nice and tight. Might surface area of the coals be another reason that smaller is better? It seems that coals would initially be lit primarily on their surfaces, and small coals have a greater surface area to volume ratio than do larger ones. edit: surface area