Why does OS 11.3.1 for the iphone 7 require 67% of my devices memory? Surely this is a mistake?

brunojkav,

Your "Other" category is very high. It is used for cache storage and temporary work files.

A restart will usually clear it up:

Restart your iPhone, iPad, or iPod touch

If no help, consider an iTunes sync. It will sometimes reduce the "Other" category of storage.

Good luck.

That large an "Other" is indicative of storage corruption. You can recover by restoring the phone using iTunes.