Google announced that you will now have 15GB of storage to use across Drive, Gmail, and Google+. Previously, you had 10GB for Gmail and 5GB for Drive and Google+ photos and they were counted as separate entities. Now, all three services will be counted as one for storage purposes. This means that if you don't use Gmail much, but you use Google Drive a lot you'll essentially have more space for files in Drive.
This change will be rolling-out over the course of a couple of weeks so if you don't see it right away, it's coming to your account soon.