Sound like word vomit.

Her comments were insensitive, yes, but as someone who can be very socially awkward and prone to blurting things out when they are nervous...this may be the case. Is your girlfriend like this? It sounds like she is also not very experienced sexually. Considering that she still gave you head, she probably doesn't have an actual issue with your dick.

The first time I had sex with my (now ex) boyfriend, he finished quicker than I expected and didn't tell me that he came, and I blurted out, "Oh! You're already done?" He turned red, and I wanted to crawl under the couch we had just fucked on, but we both got over it. I still enjoyed sex with him, a lot--it was just different than my other (limited) experiences had been.

As others have said, just talk to her about it. Let her know that comments like that make you feel insecure, but don't get angry or accuse her of being an asshole in the process. I understand why they bother you but don't think her words were purposeful or fully thought out.