I think it has more to do with the fact that GPU's memory accesses aren't cache coherent with the CPU, so a larger L2 doesn't really add much to the table.
Generally DMA to/from the GPU is cache coherent (either via DMA sniffing for cache invalidation or software managing regions for DMA, e.g. marking relevant PTEs as nocache).
So accesses are _coherent_, but the cache is simply irrelevant (or even more costly, if it's using snooping).
