Promiscuity is not the cause of your girlfriends vaginal cavity being, perhaps, on the generous side. Unless she has been having sex with horses, or some other such monstrously proportioned animal, the size of her vagina would not alter to any great degree.
Childbirth is usually the cause of what some would term a "loose" vagina. Kegel exercises are a very effective way of strengthening the internal vaginal wall muscles. If a girl does these every day, she will gain amazing control over those internal muscles. If you have ever watched some of the bar girls in Bangkok firing ping-pong balls from their vagina, you will get some idea of what is possible by being able to control the internal muscles of your vagina. A girl with excellent control can literally squeeze your penis. I do my exercises religiously, every day, without fail. It requires no equipment, and you can do the exercises sitting in your office chair, without anybody noticing.
There is nothing that you can do to help your girlfriend strengthen her internal muscles, or anyway of effectively reducing the internal dimensions of her vagina. All you can do is to not make her feel self-conscious about this. If you keep going on at her about it, you will make her feel very unhappy, and if she was molested as a child, it may have some psychological throwbacks to a period of her life which she may be happier forgetting about.
If you look on the Internet and do a search for Kegel exercises, you and your girlfriend can read together about it. These exercises will give her great control over both her vaginal muscles, and her anal muscles. If she does them properly, and regularly, then sex will become a great deal more pleasurable for both of you. Given that these exercises also help her gain control over her anal muscles, if the two of you wanted to experiment with anal sex, these exercises would be very helpful in that direction as well.
Her past relationships are obviously not something that concern you unduly. Good for you, as what she has done in the past has no bearing on how she is now. If the two of you love each other, and just accept that you each have a past, and all that really matters is the present and the future.
As far as sex is concerned, there are so many other things that the two of you can share together, that will provide an immense amount of pleasure. Penetrative sex is not the only activity that you can enjoy, and indeed, many people derive more pleasure from other activities than the actual act of vaginal intercourse. If the two of you experiment on giving pleasure to each other, you can find many ways of having an immense amount of fun, which helps to reinforce the love you feel for each other.
Never be afraid to discuss things with her, but as I always say, a conversation consists of listening as well as talking. Put her mind at rest, and don't let her feel worried or have any doubts about how you feel about her. If you love her, make sure she knows it.
I hope this helps both of you enjoy your love together.