With 3d acceleration disabled, you're using Microsoft's in-guest software renderer for your Direct3D graphics rendering.  So the "12GB" of memory that they display there is really misleading and just notes that their software renderer can use that much memory.

When 3d acceleration is enabled, then Microsoft starts using our Direct3D graphics driver and is subject to the memory capabilities of our virtual graphics device.  But it will really be running on your host's GPU in that case.  I'm comparing us to an integrated graphics card only in terms of the layout of our virtual PCI device.  We will still accelerate your the VMs graphics rendering on your host's graphics card regardless of whether you have an integrated or discrete GPU on your host.

The 4MB that it displays there for "VRAM" is also misleading, because it can only be used for your primary display output, and not for general graphics rendering.

I'm not sure what specific application you're running that isn't seeing a benefit from enabling 3D acceleration in the VM, but possibly it requires a version of Direct3D higher than what we support.  We should be able to support up to Direct3D 11 depending on your host configuration, but if an application requires Direct3D 12 then it may be running on Microsoft's in-guest software renderer.