Managing personal Outlook.com account settings, security, and privacy
Changes to Microsoft 365 email features and storage https://support.microsoft.com/en-us/office/changes-to-microsoft-365-email-features-and-storage-e888d746-61e5-49e3-9bd1-94b88e9be988
Changes to cloud storage
Starting February 1, 2023, cloud storage used across Microsoft 365 apps and services will include Outlook.com attachments data and OneDrive data.
It's their new retarded policy. Outlook attachments total size now counts towards Microsoft Storage 5 GB limit. It's likely an attempt to push free users to pay for subscription, delete email+attachments or leave. Greed is what it is.