Having just purchased a 21" over the 23", I can tell you I rationalized it by comparing to my BGE Large. The 21" has more surface, more capacity because of the extra levels, and is only marginally smaller than the 23". But I also knew that I will eventually be buying the 32" (or larger) to handle my massive cooks. I'm cooking a 12 lb brisket today, and when that's all done I'll post some pics of that cook plus my 1/2 lb burger cook from last night (9 patties). That should help a bit with scale.