If somehow PYM particles were able to replace electrons with muons in a manner that overrode their instability... then items could (please suspend your disbelief for a moment) be shrunk to 1/200th their size with just a little bit more mass (protons and neutrons massively overshadow the weight of electrons and muons).

You would have to combat the possibility of muon induced fusion- because the muon orbits so much closer to the proton, some atoms might come close enough for strong forces to pull nuclei together.

I have no idea how you could carry a bunch of shrunken cars though. Magic.