The only 5200 offered was a GeForceFX 5200 Ultra. It was the low-end option for most of the G5 models with AGP. That card had 64MB VRAM.
There are two possibilities:
1) Your card is one of the few nVidia models that worked out-of-the-box with PowerMacs. If so, the problem is in the OSX driver kext. It's not allowing for the 5200 to have different VRAM sizes. However, I suspect it's the next possibility.
2) Your card is a PC version flashed with the Mac ROM from the 64MB Mac 5200. To see the full VRAM, the card firmware would have to be edited. Another complication: the Mac version had ADC + DVI, vs the VGA + DVI that your card probably has.