I simply can not understand why any woman would want to deny having sex with a guy that has a bigger package than their husband/boyfriend if its actually true. Your guy surely knows he's not your first lover, there for he must know that even if you have only ever had sex with one other guy it's only a 50% chance that he is your biggest so what's the big deal. So, if he asks the question, you maybe should ask do you really want the explicit details of my sex life but just remember that once I've told you what I did, it can't ever be unsaid. If he still wants the details tell him its ok and you will tell him, but that you want the same details of his past conquest in exchange, one for one swap.
Tell him you want the full details of how far he got and how he got there with every girl right from the very first time he fingered her and that you will give him the same amount of detail he gives you. Maybe even make a game out of sharing your sexual history with each other
Don't be shy, tell him every detail of how you seduced or got seduced by each guy, start with the first time he kissed you and how it made you fell all the way through to your orgasm and even to where he cum in or on you, how much he cum, how good he made you orgasm. Don't be shy or embarrassed after all he asked the question not you.