Mail downloads your whole mailbox, and one setting changes that

Mail on a Mac is not a window onto a server. It downloads messages and, by default, their attachments, and keeps them until you tell it otherwise. On an account with a decade of history that is one of the larger folders in your Library.

Where it is

du -sh ~/Library/Mail 2>/dev/null
du -sh ~/Library/Mail/*/* 2>/dev/null | sort -h | tail -10
du -sh ~/Library/Containers/com.apple.mail 2>/dev/null

Each account has its own folder, and inside it a folder per mailbox. Reading the sizes by mailbox usually identifies the problem immediately, and it is almost always one mailbox with large attachments rather than the messages generally.

The two settings that matter

1. Download attachments. In Mail settings, per account, choose to download all, recent, or none. Setting recent or none for an archive account changes the trajectory permanently.
2. Keep copies for offline viewing. The same screen decides whether full messages or only headers are kept locally.

Neither deletes what is already there. They change what happens next, so pair them with a one time cleanup.

Removing what is already downloaded

Select a mailbox and use Mailbox, Erase Junk Mail or Erase Deleted Items for the obvious wins, then Mailbox, Rebuild for a mailbox whose local copy is larger than it should be. Rebuild discards the local cache and refetches from the server, which for an IMAP account is the supported way to shrink it.

# Quit Mail first, then measure again to see the real result
du -sh ~/Library/Mail

The separate downloads folder

Attachments you have opened are copied into a Mail Downloads folder that is not cleared when messages are deleted. It is the single most common surprise here and it has its own guide: Mail attachments and the downloads folder.

If you use more than one client

Each client keeps its own complete copy, so a Mac running both Mail and another client for the same account stores that account twice. Outlook's profile behaves the same way and is measured in the same way.