Messages take up 9.8 GB. How do I get this number Down?

On my iphone, in the usage tab my messages take up 9.8 GB. This is a huge amount of space and yes I have deleted every single one of my messages and it has not gone down. Is there anything else I can do?

Let the battery die and reset the device

Try moving the video files to a different storage and empty cache and restart the device. Should that fail, then backup all important files and do a "hard reset" of the device.