The current maximum allowed space is 6.8 GB out of 7.95 GB on standard dual-layer DVD. That missing 1 GB is taken up by a DVD-Video partition containing anti-piracy security. Microsoft has apparently reduced the space needed for that partition, allowing developers to use the space now.
Unfortunately, that space is most likely on the inner part of the disc, which is also the slowest section to read from. So developers may gain 15 percent more space, but they may also have to account for slower read times from the new storage space.