Men like big breasts, therefore women evolve larger breasts. Its an evolutionarily solid argument, assuming of course that male preference exerts any pressure on the evolution of female secondary sexual characteristics.
There is experimental evidence (based on questionnaires) that show that men find female breasts visually attractive. (Would it be appropriate to add a "Duh" here?) A relatively new study titled "Eye Tracking of Men's Preferences for Female Breast Size and Areola Pigmentation" examined how men make these visual judgements. The researchers used eye-tracking technology to test if men visually fixated on larger breasts, and for how long, and if areolar color altered male attention.
The results? Well, I think a friend of mine said it best: "Groundbreaking science - men like boobs."
Results of the experiment showed that men rated medium and large breasts as more attractive than small breasts. They also rated dark and medium areolar pigmentation as more attractive than light. Not surprisingly, breast size had no effect on visual fixation. Basically, if there were boobs to look at, men looked at boobs.
Here's the link: http://www.ncbi.nlm.nih.gov/pubmed/20169468