The mini CM9 has nothing to do with the device itself, but instead the actual custom ROM. Basically is is a lesser version of CyanogenMod which has been put together by the CM team, plus the Android Open Source project team, as well as added contributions from others.
All up, it’s a great ROM, but not to be compared with it’s big daddy version. You still will get some good features on the mini CM9 and it’s really smooth. It couldn’t run much better.
Some of the features you will find inside are the Nova launcher, Opengl rendering, LDPI friendly, App2SD, KSM support plus a bunch of extra settings including added extra brightness functionality.
Download the Mini CM9 ROM from this XDA thread.