I've never sat in a Mini Cooper - I have sat in a past coworker's old Mini (right hand drive tot, it was weird going to what's usually the drivers' side to get in) and that thing was tiny. I guess that's how it got the name
The Cooper is about the size of my GTI, and the GTI looks small but once you get in, you realize it's quite roomy. More often I'm hauling stuff rather than people, but you can fit a ton of crap back there. I was in Chicago a few weeks before moving out of the parents' house into my own apartment, and decided to stop at Ikea before leaving to pick up some decent looking furniture on the cheap (since we don't have an Ikea in St. Louis) and was surprised at how much flat pack furniture I could fit back there.
Whenever I'm in Europe, I frequently see families driving around in Golfs and Minis and other similarly sized cars - you never see that in the US. Everyone here's gotta have the biggest SUV made for their family of 4