jQuery Mobile, Backbone & Bootstrap for a mobile app? It makes me dizzy just thinking about the amount of code the mobile phone will have to download/execute.
Backbone is 18k minified (uncompressed)
Underscore is 13k minified (uncompressed)
Bootstrap's js is 20k minified (uncompressed)
jQuery minified is ~32k minified & compressed
and jQuery Mobile the heaviest of the lot is ~81kb minified & compressed.
So how dizzy does that make you?
jqMobi is imho the better alternative[1] for single page mobile JS apps at the moment. But not because of the size, but because it works better.
[1]http://s2.diffuse.it/blog/show/14823-Mobile_HTML_Frameworks_...