Storing money in a singularity

I grabbed this ad from a red line train:

I would LOVE to have a wallet that reduces my wallet size by 200%. Think of all the crap you could walk around with. Does it reduce the mass as well as the volume? Would I have a wallet that's much bigger on the inside than on the outside, but weighs like 4 tons, or is it more of a portal to a multi-dimensional space that you can just put things in? Like a TARDIS? Two Daleks can tow the TARDIS, even with all of the stuff it has inside, so is this like that? Can I put like a TV in there?

I think Cambridge, being home to the best engineers in the world, is probably a likely place for such a transdimensional breakthrough to make it to market.