"For what it's worth, my base weight's usually significantly different for a 2 night trip vs a 10 night trip. Even if everything else were the same (which it's usually not, see below), 12 extra pounds of food would normally push me up to a different pack."
Agreed. I usually only do overnight trips when the weather forecast is good. A 10 day trip means I pack for every kind of weather imaginable, including extra food and water if I should get caught by a swollen river or whiteout. This means my tent, sleeping bag, clothing, stove and fuel choice are different, as well as the extra food. For an overnight, I would not even bother to carry stuff like toothpaste, toothbrush, washing supplies etc..., and may just throw in a Frogg Togg just in case. Longer trips I will take a more durable rainjacket. So to me, base weight, skin-out, total weight and all that nonsense varies by trip, and also by whether it's just me or me and my partner.