I think it's gotta be big, to be worth it. The X3D dies are tiny and the additional SRAM is flanked by unused silicon to keep the package a uniform shape.

The lowest end Radeon 7600 still has 8MB of infinity cache.

Giving an APU 64MB of cache would be a mistake at current performamne levels, giving it 4MB might not be worth the time or silicon or the 4MB SRAM die floats away from the wind generated by the place and pick.

That would be irrelevant, tens of MB of cache do not cover for Gigabytes of slow memory for textures. Modern games use 4-8 GB of RAM, mostly for HD textures. Screen buffers are already in the cache.