How to reduce Mail folder size on MacBook Pro?

Re: Why is my Mail folder so big?

Deleted Messages may still be being counted ...

View deleted emails in Mail on Mac - Apple Support

Eg: says:

:Depending on how you set options in Mail settings, you can view and retrieve messages after you delete them."

and

"Deleted messages are available until they’re permanently erased"

Delete emails in Mail on Mac - Apple Support

Eg: says:

You can: "Permanently delete emails

1. Go to the Mail app  on your Mac.
2. Do one of the following:

• • Choose Mailbox > Erase Deleted Items, then choose an account.
  • In the Mail sidebar, Control-click a Bin mailbox, then choose Erase Deleted Items.

You can set the “Erase deleted messages” option in Mail settings to have Mail permanently erase deleted messages for you at a specified time. See Change Mailbox Behaviours settings."

After permanently deleting emails (if needed) in the Mail app:

You may need to Restart your Mac, in order to see the changes to Storage Amounts.

Thank you for your efforts, for checking all that, and for replying. Surprised at that result! Could it be something to do with an iCloud sync / back-up ?

Else:

If there's nothing in it you want, you could try emptying the Mac's "Bin"

(ie: in the dock, right at end of the icon line-up)

Again, may need "Apple menu / Restart" to see any results from the change.