Hi Mikkel,
The amount of mailbox storage available to a user is determined by the user subscription license:
1. Each Exchange Online Kiosk user receives 1 GB of mailbox storage. The maximum mailbox size for a Kiosk user is 1 GB.
2. Each Exchange Online (Plan 1) user receives 25 GB of mailbox storage, which can be split between the user’s primary mailbox and the user’s personal archive.
3. Each Exchange Online (Plan 2) user receives 25 GB of storage in the user’s primary mailbox.
Administrators can use Remote PowerShell to reduce maximum mailbox sizes for some or all of their users. Based on this situation, I suggest you check which Plan you are using.
Thanks,
Keats Ge