Mailbox migration from on prem to exchange online have a huge space increase

Hi,

My 1st time writing here.

I have a client that I'm migrating from Exchange on-premises 2010 to Exchange online.
I'm doing stage migration and everything is working just fine, but when it finishes syncing it has a lot more space than what exchange on premises shows.

For example, user1 has 89GB mailbox on-premises, and in Exchange online has 106GB!
So any license is not compatible with that user.

I'm asking for help on how to resolve this issue!

Thanks!