That’s “normal”. iMessage will compress the video down to almost thumbnail size depending on how fast the cell connection is. It always compresses video and panoramas and big pics.
If you want to send an original resolution image/video to someone, don’t use iMessage.
Ive had decent luck using a video compression app downsizing my video clips to 720p and 2500kbps. The resulting file size is about 1/6th to 1/10th the original and still looks good on the iPhone and iMessage won’t crunch it down farther.
Also try to keep your clips sent on iMessage less than 30 seconds- otherwise it’ll compress it heavily