My girlfriend says my dick is small, i'm 16 and 6 inches erect.?

she wanted to know how big i was erect and i told her, all she had to say was "i was expecting something bigger", she's 15, and a virgin, i know she might be 1 year younger, but still, i'm rattled. i feel like dumping her now. and i've explained to her many times already that it's average size etc. she still thinks its really "small". help? advice? thanks.