Hi Scott,
Your girlfriend feels insecure about her breast size. I'm a C-cup, and I enjoy being with women of any breast size. I think they are all beautiful, including A cup size. In fact, I find small and perky breasts very sexy. What matters in how a breast size suits a woman is who they're on. Most petite women would look weird with big boobs.
To me, the lifestyle is about enjoying variety. Everyone may have a favorite size/type of a body part (or perhaps have no favorite), but can still enjoy variety in their partners. I'm like that. My husband is like that, too.
We've been with couples where the woman had really large natural breasts, and it didn't make me feel insecure at all. In fact, I very much
enjoyed the differences between the two of us.
Another common size insecurity that is exactly like the one your girlfriend has, is the way some men feel about being naked in the same room with another man who has a bigger penis than he has. Many of these guys also feel insecure about that bigger penis being in
his wife or girlfriend (comparison thing again). Just like an insecure man might worry, "Will my girlfriend still like mine so much, after she's experienced that big one?", your girlfriend may be feeling
exactly the same way about you being with a woman with bigger boobs.
My husband and I have run into these insecure guys in swapping. I actually had one man say to me (the last time we played with a couple, actually), "I don't have anything for you". He meant that just because of my husband's size, he had nothing to offer me sexually! What a dumb thing to think.
In reality, I was looking
forward to experiencing his penis for a variety of reasons. There's that variety word again.
In the very same way, if you two are with a couple where the other wife/gf has bigger ones, her husband may be really looking forward with anticipation to experiencing your wife's smaller and perky breasts.
If a couple with an A-cup woman got together with us and seemed to be having some sort of emotional drama going on just because my boobs were bigger than hers, it would really put a huge damper on the whole experience (if the experience was even able to proceed). That's what happened with that couple I mentioned above. (It didn't work out.)
Confidence and positive attitude go a long way in making a person sexy and attractive. Your girlfriend will do well to work on her self-confidence and learning to find her own breasts beautiful and desirable.
Are you two new to swinging? You might want to continuously reinforce her confidence with compliments about her hot body. Do
you love her breasts? Does she turn you on just as she is? Let her know that verbally, and don't stop telling her. It's a chick thing. We
need to hear these things, and not from just anybody but from our significant others!
You might want to bring up the variety topic and explain to her that even if there was a woman you two were with that had a bigger pair, that doesn't mean that's your preference, it's just variety. If you aren't the insecure type of guy, tell her that you'd feel the same way if you two got together with a guy who was hung bigger than you are - that it wouldn't bother you. The more you talk, the more she'll get it that you two can play with all kinds of people, and it won't make her any less in your eyes.
I hope you let us know how it goes!