One of the things I would check is to make sure that you are putting the needle all the way in the stitch so that it is full sized before popping it off. When you're picking, it can be easy to catch the yarn just on the tip of the needle which makes the stitch smaller. Right now I'm knitting with a slubby yarn that wants to knit up tightly, so I give the right needle a little tug as I'm pulling the stitch off.
Also, make sure you are tensioning the yarn in your left hand in a way that feeds easily. I find that wrapping the yarn around my index finger makes the yarn feed more slowly and stickily (is that a word?) which tightens things up, whereas if I strand the yarn through over the index finger and under the pinky it sticks less which makes the knitting looser.