First off, does anyone go superlight and only carry a backpack?
There was an article in a print issue of BPL a while back that had a story about bikepacking a Oregon trail. It was 6 guys in 2 or tree shelters and a 5 of them only carried a backpack.
Personally I'd rather put some weight on the bike: That get's your center of gravity lower and reduces saddle and hand pressure. Also bigger backpacks tend to hit you in the back of the head when you brake on a descent.