Yes its mathematically proportinal.
If you win a game against someone rated withing 25 points of you, your rating goes up sixteen points. If you lose, it goes down 16 points. For every 25 points your opponent is rated higher than you, you get an extra point.
You cannot gain or lose more than 32 points per game.
Player A is rated 1391.
Player B is rated 1523
If player A wins, He will gain 21 points, while Player B will lose 21 points.
If player B wins, he will gain 11 points, while Player A will lose 11 points.
This system works for anyone under 2100 points.
For players with a rating of 2100 to 2400, the same system is used, except that you use 12 points instead of 16 for a game where players are within 25 points of eachother, and an addition point for every 25 points. You cannot gain or lose more than 24 points per game.
For players above 2400, you do the same calculations but you only gain or lose 8 points for a game where players are within 25 points of eachother, and you cannot gain or lose more than 16 points per game.
I know the FAQ says something similar to this, but its a little confusing, and I find this easier to understand.