When I pumped for DD1, I could pump like 8 or 9 ounces at once. I got to thinking that was normal. Then when I was nursing the twins, I could barely pump half an ounce on the left, and no more than a few drops on the right. Now clearly I had lots of milk, because the twins were growing and thriving. It must have been MORE than I was producing with DD1, since there were now two babies.
My conclusion-- you can't judge anything about your supply from what you can pump.
There's such a strong psychological and hormonal component to milk letdown. It's not just the mechanical action of sucking that makes the milk let down for baby. It's a complicated milking action that the baby's tongue and palate do, that even the best pumps can't duplicate. Plus, there's the hormonal and emotional connection between you and baby that triggers letdown. Many woman find that they just can't let-down for a pump at all, because there isn't that hormonal and emotional aspect to it. It can take a lot of practice and conditioning to have letdowns for a pump. If you're not letting-down, then you're just pumping off the milk that's stored behind the nipple, which means there's only a very small amount of it. Or you may only be having one letdown for the pump, whereas when baby nurses, you're probably getting multiple letdowns during the course of a session, even if you don't feel all of them.
Then again, my left boob ALWAYS made more milk than my right, consistently. So I think that's normal, too, to have uneven production. Nothing to worry about though. If you were just nursing, not pumping, you'd never even notice. You'd look at baby, and see that baby was growing and thriving, and you'd know everything was fine.