I've replied "they seem just about perfect for me " a time or two. Sometimes add a gentle kiss on one, pronounce it  irresistible. 
Resulted in a more confident lass in my lap.

My standard response is "More than a Mouthful is a Waste" & demonstrate !!! Seems to remove their stress. 

I haven't had this happen often.  Many years ago, my fav talked about getting implants.  I told her I might jump off a tall building out of grief, if she ruined her perfect Bs with implants.

Girls make comments about their perceived inadequate tits to me quite frequently.  I suppose its correlated to the fact that I tend to prefer mid sized tits and will take smaller and perkier over larger and droopier or saggier any day of the week.  I also favor slim and firm bodies, which often are paired with smaller tits.

I'm usually honest with my response, if they're great tits I'll tell her.  If they're not so much, I won't say "yeah, they're tiny" but I will compliment something else.  "maybe, but that ass makes up for it" or "I didn't even notice, I was too focused on your smile" or some other bullshit.

It’s hard to tell if it is ss or not, but good boobs are good boobs, whatever the size.  My CF has a preposterously large ass and tiny tits.  Not fat either, just built like an R. Crumb character.  Perfect, and I told her so.  She said that she was thinking of getting implants, and I said no way, bla bla.  She probably tells they story to 10 guys a day to build rapport.

Other girls come through with huge tits that are crappy.  Or the dreaded “floppy A’s”, etc.  technically bigger than the CF, but not better at all.

She was either fishing for a compliment, or wanted to see what your preference was. Don't get me wrong; we all have insecurities, but women who spend their entire day naked to pay their bills aren't incredibly insecure about the way they look, or they'd be in a different line of work. Plus, some guys seem to prefer smaller tits.

'the dreaded “floppy A’s”'

^ God those are the worst.  Even more so when you get tricked by a well fitting push up bra that makes em look firm.  And if she's facing away from you when she takes her bra off and you feel em before you see em, its just a humanitarian crisis at that point. 

As her boob filled your hand, she was hoping you would give her permission to experience right there and then how your dick fits in her hand.  

 This forum is for people to post whatever they want, as long as it's not political or regarding Tijuana.  We have dedicated forums for those topics.  

Was getting a dance recently.  She randomly observed, in a sad voice, that her boobs were on the smaller side.  I demonstrated that her boob exactly filled my cupped hand.  (As I remember, the technical term for this is a "no waste boob¨.)  I submitted this as evidence in favor of her boobs.  Seemed to cheer her up a bit.