First noticed by French site Mac4Ever [Google Translation], updating to 10.9.3 increases available VRAM from 1024MB to 1536MB, boosting the size of the shared memory, possibly to further improve 4K performance on certain machines.
The change can be seen in the Graphics/Displays section of the System Report accessible via About This Mac. Apple has not yet updated its support page to reflect the new VRAM limits, continuing to list 1GB of system memory as the maximum. The update was also not mentioned in the 10.9.3 release notes.
The 10.9.3 update, available via the software update tool in the Mac App Store, also included enhanced support for 4K displays and restored the ability to sync contacts and calendars between Macs and iOS devices over USB.
Update 12:45 PM PT: As noted by forum member SmileyDude, some machines with HD 4000 graphics have seen a VRAM boost as well, namely the 2012 Mac Mini, which now has a maximum VRAM of 1024MB, up from 768MB.