That is odd for her to compare you to her ex and then tell you about it. Have you been comparing her to your ex's? I really don't see where someone would think this is acceptable to do other than to either tear you down or build you up (i.e.... not as big as so-and-so or you are bigger than so-and-so).
My bf only asked that question once... how he had compared to others I had been with... it's a question loaded with insecurities. So I answered in the most honest and positive way I could. I told him that I've noticed I have a preference for a very particular shape and size... and that he is indeed within that preference.
His question was inappropriate and can be damaging to a relationship... just as your girlfriend's comparisons are inappropriate and damaging to a relationship. Ask her calmly in a curious manner why she would make such comparisons. It shouldn't really matter how you compare with her ex's so long as she is pleased by you and happy to be you. I mean, those are some of the reasons why she isn't with them anymore... right?
"The weakest soul, knowing its own weakness, and believing this truth that strength can only be developed by effort and practice, will, thus believing, at once begin to exert itself, and, adding effort to effort, patience to patience, and strength to strength, will never cease to develop, and will at last grow divinely strong."
- James Allen