Hi manfromnapa,
We understand you have some questions about the storage space being used for Messages, and we're happy to help out. When you say that the iPhone is using 6.75 GB of space, does that appear in iCloud Storage on the iPhone, or in Settings > General > iPhone Storage? If it's appearing in iPhone Storage, this means that Messages is using 6.75 GB of space on the iPhone itself, and another 22 GB is being used in iCloud. We have some more details about how storage is reported here:
What's the difference between device storage and iCloud storage?
Once we have a little bit more information, we'll be able to determine the next steps.
Have a great day.