If your opponent hasnt played many games or plays very infrequently then his rating is less accurate so it changes more.
Ratings ?????
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Hi,
Ratings changes are not symmetrical. How much your rating changes depends on multiple factors, including how "stable" or "consistent" your rating i.e. your play, is.
This is tracked in your "Glicko RD" parameter, which you can find in your "full stats" ratings page. A lower value indicates a more consistent level of play. Yours is 46 for standard live play but 291 for blitz. Your opponent's might have been much higher, thus resulting in a bigger change. I don't know the full maths behind it 
Regards,
Mark
Hello,...I'm new to this forum; kind of a novice who is trying to get back into chess, after enjoying many hours of the game in my youth. I am totally unfamilar with how to move up in the ratings, e.g. I just mated an opponent with a rating of 856 when my rating was 666 (yeah, I know,...). My opponent dropped 78 pts. to 778, while I was increased by only 12 pts. to 678. How did that happen,...??