Well, I used to be an "I" cup. I had breast reduction surgery this spring and got down to a DD. (Almost 4 lbs of breast tissue/fat removed.) In reference to the "tube sock" comment, I think that the amount that women's breasts sag is primarily genetic (and of course, related to nursing and weight gain/loss) and not a result of how tight their bras are.
I don't know how big this woman's breasts were, but I wish that people could be more compassionate. Past a certain size, breasts just BOUNCE, with the slightest movement. Before the surgery, I would have to wear a bra at ALL times - sometimes even to bed because a sudden move could be painful - ESPECIALLY during PMS. Yeah, she could have probably done better than a spaghetti strap top, but try finding a sports bra in an I cup! You can't even find a REGULAR bra in that size. I was a runner then, and I would wear two bras - one of the custom-made bras I had to order (made in a wicking fabric), underneath one of those lycra-blend sports tops. I still bounced. I hardly think I should have stopped exercising because I had large breasts (hell, i still do!).
I really doubt that she thought it was "sexy." Many women with large breasts are quite self-conscious about them (after years of having people stare at them). Having very large breasts can be a nuisance and a pain, especially for active women.