I'm probably not even considered lightweight by most here - I'm under 30 lbs but not under 20. However, I do feel that comfort is important and so is being prepared for sudden changes of weather/temps, especially as fall is slowly heading for winter.
Still I find that I am easily the smallest backpack on the trail with my Mariposa Plus, and all the coffin sized Jansports, Gregories and Ospreys going by me are often covered with sleeping pads, bags, mugs, huge GSI measuring cups, Nalgenes in various sizes, shoes boots or sandals, random bits of drying clothing, and parts of tents (or the whole darn Walmart dome tent that sleeps four). My stuff fits well into my pack and the bits I keep in the pockets are there for convenience's sake.
I do find that I run out of ibuprofen - I keep giving it to people who need it at the end of the day. Me, I just have a headache related to pollen allergies once in a while....
"In the beginner's mind there are many possibilities. In the expert's mind there are few." Shunryu Suzukihttp://hikeandbackpack.com