As you mention Common Lisp:

The implementation I'm using is really small (and contains every single symbol of the CL specs). And: as I'm doing real-world apps in CL, I'm often very happy with some (little) more complexity to gain the required performance needed.

Really, there is no real waste in Common Lisp, although this seems not to be a very popular opinion nowadays...

After spending 5 years in nothing but .Net, the CL libraries seem rather sleek and streamlined to me! I was always a little confused when people complained about the bloat in CL.

which implementation is this?

ECL.

From a pure Lisp perspective, it's maybe not the really very best choice, but from an OS viewpoint, it's simply the most convenient for my tasks (Apple, Windows, Linux, free, really fast inline C FFI).

Compressed its size can be as small as 1(!) MB on some OS.