If you check out the Glicko formula you'll realise this is caused by different RDs (rating deviations). To put it in simple terms, a player who plays often has a more accurate rating than someone who plays occassionaly. They therefore have a lower RD and their rating will change less after each game, because it's considered to be fairly accurate.
Because your opponent's rating was considered more accurate (he had a lower RD), it changed less than yours.
I (1828) lost a game to somebody (1612) I lost 19 points, he only gained 11, sounds a bit unfair to both of us