I have been inside a Sublight Sil. I thought it was quite spacious - particulary compared to my Micro-Zoid.

I interpreted the post to mean he was trying to get his "base weight in his pack" to 12 pounds, not counting water because he had to carry a lot (fixed amount)of water.

I think there is too much emphasis on the starting base weight, including water. When desert hiking you often have to overload with water for the first few days and just put up with it. The water weight rapidly is reduced as you go along.