As with the original PS3 fat (before they moved to PS3 Slim) the size of the RSX and especially the CELL was reduced, onto smaller process / node. Both chips started at 90nm and both were deduced to 65nm, as I recall. I think CELL was reduced even more while RSX stayed at 65 or 45 nm.
Same thing happened with the original fat PS2, before they went to PS2 Slim. The Emotion Engine and Graphics Synthesizer were reduced in size several times, before the single chip EE+GS@90nm (which was not actually on a 90nm process) came along for the PS2 Slim.
I probably didn't get the number of shrinks or the nodes right, but Edit:
So I do expect PS4 to have a 20nm APU sometime in the near future, without a full PS4 hardware revision.