UPDATE: Google storage quota is being unified across Gmail and Drive.
Beginning May 28th, 2013, Google will change to a new, simpler allowance for storage—one that’s combined across Gmail and Drive. We’re replacing the current, separate storage allowances—25GB for Gmail and 5GB for Drive—with a single 30GB quota that will cover Gmail, Drive and Google+ Photos.
Should users need more than the 30GB quota, they can purchase additional Drive storage using the existing ordering points, and the higher allowance will continue to be spread across all of these services.
This update is expected to roll out to Rapid Release domains beginning May 28th, 2013, and to Scheduled Release domains starting June 11th, 2013.