I don't think there's such a thing as too big or being unatractive. Certain shapes of breasts are unappealing to me, and I personally find breasts like Chelsea's to be unatractive. I hold nothing against Chelsea herself, as she is a beautiful woman, and the sheer size of her breasts is quite amazing, but I simply don't like fake boobs.
I'm one of those people who like breasts no matter how big they are. Even if a woman were bed-ridden because of the sheer size of her bust, I'd like them. If she liked them too, despite the difficulties they brought, that would be perfect. The only time I really like breasts that immoblize a woman unwantedly are merely in the realm of fiction, though. I don't wantonly pray for woman to be confined to one position by their bust size.