What happens is the messages get stored twice in your phone from Android's Native Data Base and Textra's app. Deleting your messages from Textra won't help because the Android Data Base will just restock them in Textra's storage. What you need to do is go to the Menu on Textra, go to Settings, go to About and then tap Resync Textra. This will get rid of all the messages in storage and you will then have 12 mb of User Data in Storage and about 31 mb for the App.