It wasn't that long ago that when you used the mobile internet, you would be getting a "fake version" of the site that could render speedily, despite the limited speed of 2G networks.
First it was all about WML[0], which would be processed by a proxy that would deliver the file in a binary format that would be smaller.
And even when mobile phones that could access proper HTML content hit the market, it was often still accessed through the use of an accelerator proxy[1] which would optimize the page (stripping unnecessary parts) that you were trying to access so that it could be downloaded faster.
These technologies are still in use in some places, as I understand it. But it's generally not necessary nowadays for locations with access to 3G or better.