Since the end of August, the 25GB of mailbox storage increased to 50GB. There is no price increase associated with this change as the doubling of mailbox storage is simply part of Microsoft's promise to continuously deliver value to Office 365 customers.
Customers don't need to do anything to take advantage of this new mailbox size as mailbox sizes will automatically be increased. This increase has already started and continues through November.
This increase to a 50GB mailbox per user benefits Exchange Online and Office 365 service plans, including: Exchange Online Plan 1, Office 365 Small Business, Midsize Business, Enterprise E1, Government G1, and Education A1.
Customers with a premium service plan (Exchange Online Plan 2, Office 365 Enterprise E3 and E4, Government G3 and G4, Education A3 and A4), already enjoy unlimited e-mail storage through personal archive, but now the default primary mailbox size is increasing to 50GB. Kiosk user mailboxes are doubling in size too, from 1 to 2GB.
In addition to doubling the storage of users' primary mailboxes, Microsoft has also increased the size of other mailboxes: shared mailboxes and resource mailboxes are both increasing to 10GB (more than double), but the size of Site mailboxes remains unchanged.
For complete details about mailbox types and sizes, read the Office 365 Service Description.