After increasing the storage capacity for SharePoint Online (for SkyDrive Pro) from 7 Go to 25 Go few days ago (see http://blogs.office.com/b/office365tech/archive/2013/08/27/skydrive-pro-increases-storage-and-ease-of-sharing.aspx), Microsoft has announced that the mailbox size is also increasing from 25 Go to 50 Go – see http://blogs.office.com/b/office365tech/archive/2013/08/29/exchange-online-mailbox-sizes-are-doubling.aspx
Enjoy with more storage capacity 