Personally I don’t see any issue with the size of my CSS (Foundation or Bootstrap), even when it may appear large to other developers, but then again I don’t build resource heavy websites.
For example i’ve seen developers moan about the size of CSS, I then visit the websites they build and they take an age to load - websites overloaded with images/js and all these plug-ins, animation everywhere, choppy websites when you scroll.
As for responsive/non-responsive, for anyone serious about web development, you should focus on one website, make it responsive and forget about about .mobi websites, unless there is a specific reason for having .mobi, in terms of your users, although I can’t think of any reason why your end users would prefer a watered down .mobile version