"You're so lucky!"
We don't know how to respond to this. "Sure, I love that my genetics gave me lucky big boobs." Are we lucky that our breasts attract a lot of sexual harassment and judgement? Are we lucky that it's hard to find clothes and bras that fit? Are we lucky that large breasts can be hot and painful and cumbersome? This can be a very confusing statement for someone who doesn't like their large bust and even if we're completely happy with our breasts they still require effort so please don't tell us we're lucky.
"Oh, your poor back!"
Millions of women with large breasts don't have back pain so don't assume that we do as though our breasts are a problem. Most women find that with proper bra fit (which invariably means firm band and smooth cups) that they don't get any back pain.
"Can I touch them?"
No. No you can't. I am always amazed by this one. Touching another person's body, any part of their body, is at their invitation and consent. Just because large breasts may be a novelty to you does not mean our breasts are an amusement park.
"I'd hate to have breasts that big"
Well then, lucky for you that you don't. This kind of unwarranted comment is such an insult. You don't have to like or want larger breasts but you also don't have to share your opinion with other people, especially those people who have larger breasts and may actually like their body. Say this to someone who is struggling with their body image and you've inflicted a really painful and damaging blow so just don't say it.
"How do you sleep/run/swim/lie down/function with those things?"
My first instinct is "None of your business" unless it's someone struggling to come to accept and manage their own large bust. Every person has their own physical story and manages their life accordingly. Large breasts can require some management for how you lie down and run etc, but how each person manages that is their own business and does not need to be explained to others.
Even if you know someone really well asking these kinds of questions or making these remarks can leave a person feeling self-conscious. Every woman is SO much more than her breasts so don't reduce someone to this one physical feature with irrelevant, invasive and stupid boob comments. xx