Check another post about storage used by the (android) app. Maybe add iOs in the title of this topic cause Android has its own topic: https://forum.inaturalist.org/t/limit-cache-size-or-allow-cache-clearing-for-apps/2059
Logout/Login or reboot should work on an iPhone:
https://forum.inaturalist.org/t/inaturalist-storage-what-is-the-safe-way-to-reset/25457/4
https://forum.inaturalist.org/t/data-storage-on-my-phone/7450/6
https://forum.inaturalist.org/t/inaturalist-using-lots-of-storage-on-iphone/3316/7
https://forum.inaturalist.org/t/iphone-app-using-insane-amount-of-storage-space-help/15723