Link to their css file: http://us2.admin.mailchimp.com/release/5.5.1/css/screen.css

Minified. About 215k. Taking a glance, it looks about as clean as it can get, and any more optimization would probably make it much less flexible.

It does appear to have an exorbitant usage of !important, but I won't judge since I don't know any of the ins and outs of what their team has to deal with.