Hi;  Recently I lost a match and of course expected my rating to go down, but not as much as it did which was to 1039 from 1200, and it's only one game...

I also don't know the underlying computations, but what I know is that the less number of games the players played, the more the change in rating.  Since you and your opponent played this very first game, the change was large.  If you both have played 1000 games, then the change will be small.  You can recover that rating by playing more... and I guess only a few people look at ratings.

Many thanks; I think I understand...