What email client are you using? Maybe there are masses of emails including all their pictures and attachments stored in the Library's Mail folder if you use Apple Mail since many years, especially with POP3. Or somewhere else in the Library if you use a different client. Airmail for example stores in Containers/it.bloop.airmail, Thunderbird stores in Mozilla...
I use Safari to get onto my iCloud mail.
I had previous issues with my MacBook Pro when using the Mail app. It did the same thing that's happening now but the "other" was all old mail.