Two separate points. Yes, weight conscious backpackers want a dense/light mAh/cuin mAh/oz product, but also just-enough power. But I was also saying two batteries for redundancy. In general across the entire Anker product line, the larger packs tend to be denser, but there a very few sweatspots, the Powercore+ Mini, the Powercore 10000. The larger Anker powerbanks like the 26800 and to a lesser degree the Powercore II 20000 have roughly the same density as the Powercore 10000, so two Powercore 10000 is a little lighter than one 20000 (small % difference) but from a redundancy perspective you'd be better with two as its harder for two to fail than one to fail, especially if kept separate. So if you combined the just enough with redundancy approach then it could be: a phone with a spare battery no Powercore and swap the phone batteries and recharge when sunshine allow, or one or two small Powercore+ Mini, or one or two Powercore 10000.
From the forums seems backpackers prefer the 15W, a little lighter. Personally I prefer 21W because it can make use of weaker sunshine.