As an exercise, I've been thinking about what gear I would choose in order to have the smallest pack. There's a lot of crossover between SUL and having a teeny pack, but what if pack size were what you were mostly optimizing for?
I think some good conditions to assume for this exercise are:
-Nighttime temps around 40-50 F
-Potential rain (needs something to serve as a rainjacket and a shelter)
-Not relying on found materials for a sleeping pad or bag replacement, i.e. 'forest duff' is not an appropriate sleeping bag
-Cook kit isn't necessary but food size differences should be considered for no-cook food vs. dried food that needs hot water
-Try to minimize gear carried outside of the backpack or account for its size with the rest of the gear
This may seem like a silly question, but there are times when compact gear matters at least as much as weight. I'm thinking about times like traveling on a plane and having to worry about baggage size maximums (along with all the other junk you might need for your trip), bike touring where air resistance due to larger luggage is a considerable factor, and just plain fitting into that awesome daypack you couldn't pass up. What volume do you think you could manage?