Previously, the max VRAM was 1024MB. The increase may improve performance when the notebooks are used with 4K displays.
Apple released OS X Mavericks 10.9.3 on May 15th bringing improved "4K display support on Mac Pro (Late 2013) and MacBook Pro with 15-inch Retina Display (Late 2013)".
You can note the update by selecting 'About This Mac' from the Apple menu then clicking 'More Info...', 'System Report', and 'Graphics/Displays'.
Read More [via MacRumors]