One bigger than other

FarmWife 🌾 • 2015 👧🏼 2017 👼🏼👼🏼 2018 👧🏼 🌈 2020 👶🏼
How come one breast produces more milk than the other. I switch every other breast when feeding, pump each the same..? I get 3 ounces if lucky on one and five or more on the other.