Isn't it just a matter of putting more/less plastic around the contacts to make it fit the different sizes? When I got a new SIM card a few years back, I could just pop what I needed out of the full-size card. Going back from a smaller to a larger size you'll need a plastic 'adapter', but that's just like SD cards and all the different sizes found there.

Edit: oops, looks like three others already said the same!