Hello!
I might be able to provide a likely answer.
There was a significant study published in the Journal of Socio-Economics in 2008 that found a significant link between higher status in men and a preference for smaller breasts. One of the authors posited that that the link might be linked to ease of access to food.
Renaissance art, particularly when it comes to who exactly is depicted in said art, was highly class dependent. Wealthy patrons would often commission pieces to feature women who were close to them, or artists would use women they knew as models. In both cases, the artists being supported by a patron would likely see a lot of noble women. Women who are associated with financially stable men who have access to plenty of food. Which means that according to the study, they would have a predilection for small breasted women rather than women with large busts.
We also know that, at least in England, during the Renaissance, there were some peculiar ideas around about what happened to a child when he sucked at too great a breast. Here is a passage from a period book called The Art and Science of Preserving Body and Soul by John Jones it appeared in 1579.
"Furthermore, through sucking of great breasts, the children are made flat or crooked nosed. By the nurse to be helped, if she will always when the child sucketh, depress her breast with one finger above the teat, and the other underneath."
So, at least in England, there was this folk belief that the best breast was one that was not too big.