Low quality video sent via iMessage when on WiFi only

Hello MattPete,

Thanks for reaching out to Apple Support Communities. We see you have some questions about low quality when sending messages through Wi-Fi, but not cellular data. Have you tried getting closer to the router, or resetting the router, to see if this happens after? As this is only happening with your Wi-Fi, we'd recommend troubleshooting a few steps with your network, or try with another network, to see if the issue remains.

Also, be sure your iPhone is updated, with help here: Update your iPhone, iPad, or iPod touch - Apple Support

Regards.