Hey man,
I'll try to impart some useful advice without gettin' preachy, if that is possible. Fuck it, I'm gonna preach as your virtual mentor here, as a qualified pimp who's been around a while and made plenty of mistakes (and learned from them)
First quit sweating about what your lady did or did not do before she met you. THIS is cardinal rule number one. Trust me...this is FACT and the ladies here will back me up.
Second,if you are in a healthy and loving relationship, and she's giving you plenty of ass, you should be stoked about that. It's what most people in this life are after with their partner. If you got it, keep it rolling and make her fucking happy not just in the sack but also in other aspects...do nice things for her, treat her with respect and love and by all means give her lots of good boning, she'll return in kind and you guys will live happily.
Third and REAL IMPORTANT: Just forget about what she did before. You don't want to know the truth. In fact, if she did sleep with dudes of different size and skill and she's with you, there must be a good reason for her being with you now, right? One, your dick is making her perfectly happy. Two, she has some experience elsewhere hence she does not need to explore that again cuz she's made a choice and is happy with YOU dude.
It is bad, bad, BAD for you to dwell on past experiences of hers...NOTHING good comes of it, it will eat you up so just fucking STOP. Be glad/happy/rejoiceful that she's the one and you are happy with her, and she also chose YOU. And by all means don't fuck it up by perstering her about her past cuz I GUARANTEE that will only piss a woman off BIG TIME and have a greatly negative impact on your relationship.
Finally, mix up your repertoire more. Use some cool sex toys together...get her into ass play, become the world's number 1 cunnilinguist (sounds like you're on track for that), and YOU be the one to open up her to new/better/more sexual experiences which rock both your worlds. There is so much to do beyond just fucking Bro! So step up and be the MAN!
And, from possibly the horniest man on the planet (me), remember that a relationship is WAY more than sex and dick size.
My word is gospel truth!