Apple's Mail app doesn't provide a lot of control over how much storage it uses.
https://www.howtogeek.com/247435/how-to-free-up-space-used-by-your-iphone-or-ipads-mail-app/?utm_source=dlvr.it&utm_medium=blogger
https://www.howtogeek.com/247435/how-to-free-up-space-used-by-your-iphone-or-ipads-mail-app/?utm_source=dlvr.it&utm_medium=blogger