Brad Pitt fucked her with his tiny one and you know what, she loved every minute of it and loved him. Just looking at him made her cum. But he ditched her because hes Brad Pitt and he can get anyone he wants.
All jokes aside, i get it, you think if your the biggest then your probably the best lay shes ever had and she'll never leave you but its bullshit. Your not the first or the best at anything and even if you are and you guys eventually break up, she'll eventually find someone better. If you were a girl and you fucked some humungous guy who was ugly, with no job and who lived at home, and then you met some rich good looking guy who was slightly smaller and the sex was 95% as good as with the big guy, who would you choose or think more fondly of? Would that 5% sexual difference seem important at all in comparison to the 1000% increase in desire the other guy gave you? Does it bother you that the ugly guy sitting at home laughs to himself "oh I'm so good with my big dick" whilst the rest of the world thinks "what a loser." She doesn't give a shit about his dick.
Just accept that shes a girl living her own life and shes entitled to have enjoyed sex in the past with all sorts of sizes, just as you probably have as well with women and their various differences. Does it matter? no. A smaller guy might've been better than you, you'll never know and its honestly not irrelevant.
Insecurity is bullshit and it will corrode your relationship, measure yourself against yourself (career, looks, attitude) and not your girlfriends past. Be good to your girlfriend and be the best you can be