According to this japanese article, Sony have both financial and technical troubles creating smaller Cell and RSX chips, and NVIDIA is being against a Cell+RSX merger so they can't do that either. So they are basically locked at the current sizes, for who knows how long. Maybe it will stay like this for the rest of the PS3 generation.
I guess this is one of the reasons why Microsoft didn't use NVIDIA during this console generation like they did with the original Xbox, as I do remember an article about Microsoft being very unhappy with NVIDIA as a console partner.