I tend to agree w/the above. The 2 girls I've personally known & tickled on their breasts had small breasts, which is fine! One was extremely ticklish on her breasts, and could not stand having them touched. The other was slightly ticklish on the sides of her breasts.
Now, I did know one girl in college who had extremely large breasts who said she didn't like her breasts being touched because it tickled, but since I never had the chance to verify that I don't count it!
My personal experience is that women with smaller breasts tend to be more ticklish on their breasts.