This of course doesn't mean that small breasts are evolutionarily favorable (and I'm sure someone's done studies correlation breast size and milk production and infant survival) but that for whatever reason those populations favored a certain breast size while others favored other sizes. There is much more on play here, such as Founder's Principles, etc., and because humans have the power of reason the random factor is through the roof. This is a greatly simplified example, to be sure.
It's not true that larger breasts are better at making milk. Breast size has little to do with how much milk breasts make. It's uncommon for breasts (of any size) to fail to make enough milk to feed a singleton human infant (or even twins for that matter). If the milk supply is insufficient, then the problem has nothing to do with breast size but could be to do with 1. preexisting medical issues (including hormonal imbalances), 2. insufficient nutrition/hydration of the mother, 3. the baby not latching on properly, not suckling often enough, and similar, which usually is the result of crap advice from medical professionals about breastfeeding and what to do when facing problems.
2 is the most likely explanation in 3rd world countries, and 3 is the most likely explanation in first world countries because male dominance in medicine and marketing strategies of formula milk companies has led to a complete dearth of knowledge of how to support breastfeeding mothers and unfortunately, bad advice does result in mothers being unable to produce enough milk to exclusively breastfeed. 1 happens, but isn't that common, but medical professionals that fail to help mothers to breastfeed will blame 1. when really it's 3. (My advice for women in that situation is to contact La Leche League as they do have good advice and support networks)
**note (cause I know how this topic sometimes go down on the internet): I'm not at all against formula milk (as long as it's a proper, medically approved formula). It's the right choice for many mothers. However, there are also lots of mothers who want to breastfeed full time but end up unable to make enough milk due to shit advice and a total lack of support, and that's why I care about this issue.***
It's possible for most women to make enough milk to exclusively breastfeed twins, as long as their nutrition is adequate and they have good advice. (In the absence of medical/hormonal issues that can impact breastfeeding or make it impossible - again, nothing to do with breast size.)
The limiting factors in producing milk have nothing to do with breast size. Anecdotal evidence is not scientific, but the anecdotal evidence (from midwives etc I've spoke to) is that women with smaller breasts make more milk and find breastfeeding easier. The latter may be because it's easier to latch the baby on. The former is an unscientific claim, but there's no smoke without fire... obviously there are plenty of small-breasted women who produce tons of milk.
Additionally, what size breasts straight or bi men and gay or bi women prefer is far more likely to be cultural than genetic, so the founding principle isn't likely to be a thing in this case. More likely, particular cultures favour various attributes in mates, and these are promoted through media and popular attitudes, and kids/teens learn them and it influences what they find attractive as adults. These change over time within the same ethnic group. For example, in Europe in Renaissance times, curvy, plump women were considered the pinnacle of beauty, but in Europe in the late 20th century, ultra-slim supermodels were considered to be the pinnacle. Cultural ideas changed, but the population hasn't changed significantly. Additionally, even if many men (and gay women, etc) in a particular culture prefer a particular attribute, they're not a hive mind and there's a massive amount of variation in preferences. There is no single definition of beauty in any culture.
Some aspects of human attraction probably are genetic. There's evidence that hip to waist ratio preferences are inborn and not cultural. Similarly, attraction to particular pheromones is likely inborn/genetic. IMO finding breasts (of any particular size) sexy is genetic and the fact that humans have them and they're considered sexy was a big mystery to evolutionary biologists, because it goes directly against what's seen in other mammals. Breasts are not a sign of fertility - swollen/lactating breasts/nipples are a sign that a female mammal is
not currently fertile.
There is a new theory that covers why female humans have breasts and hidden ovulation, why humans are bipedal, why humans evolved such tiny canines and why humans are less aggressive than other ape species - all these things are related, and it's to do with male and female hominins pair-bonding and sharing food, and males providing for females while pregnant and lactating, and females choosing males that are less aggressive with smaller canine teeth (they are more likely to form a pair bond and provide for them and their babies) and males choosing females that don't look fertile (less competition with other males) and females mating all round their cycle (males mate with them all the time, making them more likely to be the father when the female ovulates, while other males don't notice that she's become fertile). Bipedalism evolved among all this because it's easier to carry food to share with your partner when you walk on two legs.
There's a lot more detail to this theory and it's fairly new, and most people are only familiar with theories that were dominant in the 60s and 70s - it's worth reading about this theory from scientific sources if you get the chance. It's too complex for me to do it justice in a forum post. The upshot is that breasts are sexy because about 7 million years ago it was a sign of a mate that a male won't have to compete with other males for, and as long as the two of them bond and he gives her food, he has a good chance of fathering her babies and his provision will give their babies a better chance of survival. Things may have changed somewhat over the last 7 million years (i.e. a population where all the females have breasts and all the (straight or bi) males like breasts) but evolution is like that - constantly adapting to a constantly changing set of factors.
Sorry I haven't had much time to reply to this thread in a way that would do it justice. I don't get enough time what with the kids and the day job.