Using New Outlook on Windows for personal email, calendar, and contact management
Hi James Butts,
I can understand why seeing your storage change from 40 GB back to 15 GB would be confusing.
Microsoft currently provides 15 GB of Outlook.com mailbox storage for free accounts, while Microsoft 365 subscribers receive 100 GB.
There isn't a standard 40 GB mailbox tier documented in the current storage limits, so it's difficult to determine from the information available why your account previously displayed 40 GB. I suggest checking your Microsoft account's subscription/storage status for any entitlement changes. (For your reference: Outlook.com storage limits)
If you believe your account should still have additional storage, Microsoft Support can check the account-specific entitlement further.
I hope this points you in the right direction.