Are you sure that 80% of all mobile data connections in the world is like 2g? Mobile speed in developing world isn't actually that bad.
http://www.diffusedonline.com/wp-content/uploads/2012/04/pag...
http://www.diffusedonline.com/wp-content/uploads/2012/04/pag...
http://thenextweb.com/insider/2013/07/23/akamai-average-inte...
My 80% number was hyperbole, but I do think the data supports that on world-wide average, mobile connections are drastically lower, and so making assumptions about a file size downloading quickly on fast broadband is ignoring an increasingly important part of the web traffic, which are years behind wired connections in terms of speeds.