It's the Glicko rating system at work. Check some of these threads out to understand the technical stuff. Your observation is correct: the more you play, the fewer points you win or lose. You could even Wiki it for the math.
http://www.chess.com/forum/search?keyword=Glicko+RD
After you have played on chess.com for a year or two, what happens is if you beat a (example) 1600 in live chess, and your rating is say 1200, it only goes up 10 to 15 points. It doesn't make sense!!!