I purchased a MacBook Pro Retina 15.4" recently and I dual-booted it with Windows 8. On the Mac OS it says I have 1 GB of VRAM for the NVidia 650m (which it should report), but under Windows there's only 256 MB of VRAM.
Why is this?
That graphic card have some bugs under Windows and Linux, and showing the only 256MB VRAM, instead of 1GB...
Try to download AIDA64, and check. This tool is very powerful thing, hope is going to tell the true values :)