My sister has a herd of 7 or 8 "bigs" and a mini -- they have been together for years and the mini runs the show. We recently brought home a mini as a pasturemate for our TB and they absolutely love each other. As with any pairing, it will depend a LOT on the personalities of the individual horses - regardless of who is what size. I didn't realize just how lonely our TB was until we brought home her new friend.
The only thing we have encountered so far is that the mini is a very easy keeper (they are, after all, known for this) and our TB can be a bit of a hard keeper. We've just adjusted how we make feed available so that our big girl gets all she needs w/out our little one overeating.