Hello Gevster66,
Thanks for asking the Apple Support Communities for help with your iPhone. If you have large attachments in your message threads, the option to review and deleted them should populate.
If the option is not populating, I recommend the following troubleshooting:
1. Force close the settings app: Force an app to close on your iPhone, iPad, or iPod touch - Apple Support
2. Restart your device: Restart your iPhone, iPad, or iPod touch - Apple Support
3. If the issue is still occurring after those steps, I recommend making a backup before the next steps. You can make a backup in iCloud or iTunes using this article: How to back up your iPhone, iPad, and iPod touch - Apple Support
4. After your data is backed up, I recommend reinstalling the iOS using iTunes. The reason we need to use iTunes for this step is to get a new copy of iOS installed to your device. Use this article to reinstall the iOS: If you can‘t update or restore your iPhone, iPad, or iPod touch - Apple Support
Please pay close attention when you reach step 4 in the article. Once your iOS is reinstalled, test if the issue is still occurring. Thanks again and have a great rest of your day.