Women who express a bucket in one sitting are expressing instead of feeding their baby. I went back to work, had to express once at lunchtime (instead of the 3 or 4 feeds DS would have had if I had been home), and would end up with a couple of bottles full. In your case that's just all going into your baby.
If you google "nursing angel" you will find a rough guide to breastshield size too.

You may be better off just hand expressing into a clean cup whenever you can. Put the cup back in the fridge and squirt some more when you have a minute!