The Mac LC maxes out at 512k VRAM

Is this a hardware limitation or noted as there weren't larger single VRAM simms available for the original LC?

EG - there's a 768k VRAM simm (labeled for the radius pivot board) dirt cheap on EBAY ->

http://www.ebay.com/itm/SIMM-MEMORY-Module-MAC-LC-PIVOT-RADIUS-768K-VRAM-334-0026-A-MICRON-749-/370450005504?pt=US_Memory_RAM_&hash=item56408a7e00