I can guess at one possibility. Some time ago Microsoft optimized the address space usage of D3D9 on Vista - see https://support.microsoft.com/en-gb/kb/940105 It's possible that that optimization was only applied to x86 as you're not going to run out of address space on x64.
Is this extra memory usage actually causing a significant, measurable performance issue? If not I wouldn't worry about it.
If you really want to investigate what's going on, I'd suggest creating the simplest possible test program that shows the memory usage difference, and using a tool like https://technet.microsoft.com/en-us/sysinternals/vmmap.aspx to investigate how memory gets allocated differently.