Creating Mobile Version for Your Backbone.js App using jQuery Mobile
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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_...