The difference in framework "weight" is mostly about an opinionated view regarding how the non-framework part of your final application will and/or should look like. Backbone is assuming that this will look quite differently, depending on your personal style and the needs of the application. Other frameworks differ.

It is quite likely that the final LOC for lots of frameworks will look the same, assuming both framework and app developer are on the same professional level. Opinions differ on how much of that code is reasonable to put in the framework itself (and I bet a lot of that is stemming from differences on how professional/good your average app developer is).