Modest is modest, the problem is your emails. You have too many of them. The easiest way is to simply delete your emails. Emails are meant to be read and deleted, no one needs to keep all of them.

One easy solution if you do not want to selectively delete your emails is to delete and re-create your account(s). Deleting an account will delete all cached emails. Re-creating it will then re-download all the email headers (which may take a while if you have thousands), but not the whole emails. Those will get downloaded only when you open them. Presumably you will not open all of them.

There is one problem with Modest. It does not save sent emails in the Sent folder of a specific account, but in a local Sent folder cached on the phone only. That one will have to be purged manually. You can move messages from that folder to your IMAP folders if you wish.