how do we know they aren't shrinking all the other components too?
The memory is standard memory, they have no controll over it's size. In fact if they wanter even smaller memory, they would probably have to pay more for it. (compare desktop and laptop memory prices)
They cannot shrink the blu-ray drive.
The cell (and GPU) are the only main componet that Sony can directly shrink, and those should already be in the full sized PS3's, so you are not saving any money there.