It's totally non-scientific, and probably not something we can rely on in the future, but...
Right now, the highest capacity 1S packs are around 8000mah. The highest capacity 2S shorty packs (which are in a much larger case) come in around 5500mah. Presumably the need to cram more physical cells into a small space reduces available capacity since each cell has its own shell, and increasing that uses more of the available room.
No doubt this will improve with time, but it seems like requiring more physical cells has a similar effect to reducing the maximum allowable size of the battery case. Does 2S do this?