I had this problem a long time ago and never figured out why it was. However, I had it again today and I figured it out. It was because I had my laptop hooked up to an external monitor and then I went back to view Facebook on Chrome on my laptop screen. I'm guessing here, but it has to do with screen restitution resizing. What I did was close the chrome window (important*), set my computer resolution back to 1080p then restarted P.C. Vids on Facebook are now back to normal.
Hope this helps.
P.S.
Play around with your display settings pm P.C., it doesn't have to be 1080p but the higher the better for Facebook to view everything without a problem.