A fantastic improvement in SharePoint Online as part of the Office 365 Wave 15 service update is that the My Site feature is being replaced by SkyDrive Pro.
While My Site was limited to 500MB, SkyDrive Pro matches its consumer sibling by offering each user 7GB of storage.
While the storage usage can easily get away from users they can easily keep track of it by browsing to the Site Settings:
Under Site Collection Administration select Storage Metrics:
And here is your output: