Breastfeeding vs pumping

Does anyone know how much of a difference it is between the two in output?I’ve been breastfeeding my twins since they were born (10 weeks) and started pumping instead of nursing a few days ago because I wanted to see how much they were getting. I know putting them to the breast is better than pumping, but does anyone know how much of a difference it is? I obviously don’t know what they are getting when nursing. I am having to supplement with formula at each feeding except the middle of the night feeding and first morning feeding because I nurse them at those times. When bottle fed they both eat 5 oz. I will be going back to work in a few weeks and they will be taking mostly bottles so I figured it was ok to start now and get them on a routine.