Hey Tuanphan,
I just launched a real estate site and am running into the same thing with a background video. On the desktop it looks totally fine but the scaling on mobile is way off. I am guessing most people coming to my site will be looking on their phones so would love to have it correct.
Website is www.smithmountainlakerealestate.com
attached a few screen shots. What would be the best way to make sure it fits right on both desktop and mobile. Thank you so much for your time.