Rebeka,
It really varies but you can increase the fat in your milk by increasing the omega 3 fatty acids in your diet. Dieting can effect the quality of your milk. But assuming you are eating enough and getting adequate nutrition, your body is likely producing exactly what your baby needs.
That said, when you are pumping, are you pumping until your breasts are completely empty and then pumping some more? The first milk that comes out is called "foremilk" and isn't fatty. The milk that comes later is called "hindmilk" and contains most of the fat. I double pumped with a Medela Pump in Style Advanced (get a hands free pumping system like an Easy Expressions Bustier or Halter off ebay!) for at least a half hour and I'd typically get 2-3 ledowns of milk (where the milk starts spraying) during that time. If you aren't pumping enough, your milk could very well be "low fat." If your baby has bright green poop and is gassy, that is also a sign that they are getting too much foremilk and not enough hindmilk.
As far as baby size, some babies are just small. My first son has consistently been below the 5th percentile for height and weight but he has grown on a consistent curve so my pediatrician is not concerned. My second son has been around the 25th percentile all of his life and today (he's 2.5) he was in the 95th percentile for height and the 50th for weight (so he's really grown in the past few months). Your doctor will be more concerned with your baby's growth in general than in the comparison with other babies.
Good luck and happy pumping,
T.
P.S. My favorite breastfeeding resource is www.kellymom.com You can find just about every topic you are interested in there and the info is really good.