When it comes to email lists, quality is much better than quantity. On most systems it's easy to find and delete users who either haven't engaged for x days/weeks or haven't opened the last (insert number) emails. And yet, this latter functionality is nowhere to be found. Keap will actually suspend your account if you don't prune your list but these functions seem clunky or unavailable in Zoho One.
With a simple rule in place to delete users who haven't opened the last x emails, open rates and engagement will go up correspondingly, and this is not difficult to make happen from a code perspective.