Mac storage basics · Mail, Messages and chat apps
Why does a Gmail account take so much space in Apple Mail?
Short answer: Gmail exposes labels, including All Mail, as IMAP folders, so Mail may store messages more than once. Hide labels like All Mail from IMAP in Gmail settings and limit attachments.
Gmail shows labels as IMAP folders, including All Mail, which contains every message. Apple Mail can end up downloading the same messages for several folders, so a Gmail account often uses much more space than its inbox suggests.
In Gmail's web settings under Labels, you can stop labels such as All Mail from being shown in IMAP, and in Mail you can limit attachment downloads for the account. Remove and re-add the account afterwards if you want Mail to rebuild a smaller local copy.