Galaxy S9 SM-G960U T-mobile.
Mine is the same. Sends them as MMS. For one or two weeks after I installed the public release of Android 9.0 it was sending and receiving using file transfer. Awesome 58mb video sent and received really fast. No scaling or compression at all. Then it stopped and went back to MMS. Never could get it to use file transfer again. I can only guess the carrier has something to do with that.
@user42zjtely9U is correct that since the messages are sent over MMS they must be compressed and reduced in size to match the capability of that service.
One option that you might want to look at is the "Share large files" option in Gallery when you choose to share something. It essentially puts the item in the cloud and passes a link to your recipient. They click the link and see your item in all of it's glory.