Office 365 had announced a road map for a long time ago. In 2017 the default mailbox size is 100GB and it will be more than that in 2019.
The default mailbox size for Exchange Online E3 and E5 users is increasing from 50GB to 100GB. If a mailbox has been assigned a non-default quota (for any of the 3 quota values) then no change is made. Tenants will be able to increase quotas up to 100GB.
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