Hey neill, the architecture of the nvidia vram is different from the AMD one not to mention the Cuda cores.
AMD goes on core quantity while Nvidia goes on frequency. Nvidia cards were never as efficient for mining as AMD cards exept for Zcash. Its a hardware thing and i dont know the exact answer to why it is as it is. But you do have the option to mine Zcash and this is just as good if not better :)
I hope i could help you a little bit.