out of proportion... make the girl look unattractive and would also cause mega back/shoulder pain IRL.
if she had that much fatty breast tissue.. would cuase serious problems for her mammary glands and run the risk of cancer.. therefore even from a practical point.. breast not usable for babies...
Each of us may have our own personal preferences for breasts size, but a physical copy of the hentai manga "Blue Eyes" is definitely something worth adding to one's collection. After all, it is a classic! :D