What constitutes a "large" piece of wood is relative: throw a 30 pound, eight-inch dia. log on top of a lit candle and a blazing log-on-fire will not ensue. The result will be a long-smoldering acrid white smoke. Throw that same log into a good sized offset that already has about 30 pounds of coals going and it will become an inferno in no time, producing the desired sweet smoke. Chips, chunks, splits, logs, and trees can all be the source of pleasant tasting barbecue, or of ruined meat with more of an ashtray flavor. It's a matter of proportion of added fuel to existing combustion.