I think, by default, email logs are only kept for 90 days so this will restrict how far back in the history you can reach.
Having a glance down your /admin/email/sent page may shed some more light on exactly what kind of emails may be generating the excess. I think if you’ve set any categories to be default watching that can sneak up on you, or possibly a change in activity summary frequency? You can also use the filters at the top to narrow the list down, particularly the ‘email type’:
