Oh my. Lots to dig into here.

Let's start with breast size. No, breasts don't need to be big to produce adequate milk. Many women with small breasts have breast tissue that is dense with gland structure, meaning they have plenty of milk production capabilities.

About 1 - 4% of women do struggle to provide enough milk for their offspring, but this is typically a hormonal issue that has less to do with breast size and more to do with ovaries. As long as the mother's breasts increase by about a cup size during pregnancy, it is expected they will have adequate milk production.

Here's a source on the topic:

https://www.didymos.de/en/blog/small-breasts-no-milk/?srsltid=AfmBOoqepv0oQNz0TGIpPESnwNiIT2-eVP7aY7a_GHCI2Tn29O19FVMD

Yes, wet nurses used to be prevalent. And yes, they used to be hired when mothers could not produce enough milk on their own (that 1 - 4%). But childbirth used to be a lot more dangerous, so a major role of wet nurses was to provide for the infant when the mother died during childbirth.

But the primary use of wet nurses was by the upper class. Not because the mother required help producing milk but because such activities were seen as lower class. They would leave it to the wet nurse to feed the baby which also allowed them to stop lactating sooner and return to a fertile state where they could produce more children sooner.

It's also interesting you bring up dairy cows because they are a prime counter-example to your point. Over many generations, we bred and selected for dairy cows with large udders for increased milk production. They didn't produce such huge volumes of milk until we started breeding them to do so. So, they are a good example showing that animals in nature do not need large breasts to produce an adequate volume of milk, but breeding for larger breasts can increase the milk production to excessive levels. And cause back problems.