The point of frameworks is to write something well once. You might then iterate on the core application features, but since you use that code again and again it's cheap.
Many frameworks are larger than you might expect them to be because they take care of edge cases you haven't thought of. YUI for example is guaranteed to work in 6+ browsers on 3 platforms, as well as being tested with screen readers and other assistive technology of disabled users.
With regard to mobile devices, presumably the mobile devices are going to get fast enough to deal with it (my N95 deals with the YUI code on the yahoo.com homepage just fine) or we will make widgets specifically for those devices which are stripped down.
Many frameworks are larger than you might expect them to be because they take care of edge cases you haven't thought of. YUI for example is guaranteed to work in 6+ browsers on 3 platforms, as well as being tested with screen readers and other assistive technology of disabled users.
With regard to mobile devices, presumably the mobile devices are going to get fast enough to deal with it (my N95 deals with the YUI code on the yahoo.com homepage just fine) or we will make widgets specifically for those devices which are stripped down.