OCZ are one of the few companies that are 'honest' about their drives. Most manufacturers market a 128GB drive but it only provides 120GB of useable space (achieved by a con in the numbers used to calculate the capacity).
Snow Leopard, appreciating the habit of manufacturers of doing his has adopted the same accounting method so they will now report 128Gb drives as having 128Gb capacity.
Hence how you seem to have magically gained space. This is also the reason why some people are reporting huge space savings with Snow Leopard. give the full install of Leopard is around 12GB, there's no way Snow Leopard could save 20GB on that, but when you factor in that drives will also report more free space because of this change in measuring, you can see where the mistake comes from.