If you're interested in the math, google Glicko rating system. The 2nd or 3rd link is Glickman's page and has the mathematical details.
Basically it it provides a best guess for the result of future games based on past games for a given set of players. Against an equally rated opponent you should win the same amount of points for a win as you would lose for a loss.
However when the rating is new or you've been absent for a long time, it assumes the rating is less reliable and there's a multiplier so you'll gain and lose extra points to more quickly establish your actual rating. The more you play, the smaller this multiplier becomes game by game. This may have been the cause for the difference you noticed.
Can someone tell me how the points-system works? It seems I'm losing more points after a defeat against an equal opponent than I'm getting after a victory against such a player.
B.K.