Having seen a lot of portable speakers, I don't think it'll get any more compact and light than the Vol Speaker by Hironao Tsuboi.
Tsuboi designed the Vol Speaker for Kent of British American Tobacco. Quite an unusual collaboration, but the result is impressive and stylish nonetheless. This little item is miniature and comes in the shape of a knob. It also functions as one as well. You can simply twist the knob to turn it on and adjust the volume of the speaker. Unfortunately, this is just a concept at the moment, but speculations are that if this comes through, it'll be made from aluminum and be USB rechargeable. Hopefully, this interesting gadget will come into fruition, because outside of its technical aspects, it's also a really cool design concept.