Why am I *also* hurting my gf when we have sex?

Explaination: Her previous partner was bigger than me, like approximately 7-7.5NBP and like 5-5.3" girth (rough estimation), and she explained that they sometime had to stop because of pain. Two days ago we had sex for like 20+minutes, but we eventually had to stop because she was starting to hurt, we just did some couch cowgirl after a lot of deep penetration. I am just 5.5NBP and 4.8" girth (although I am closer to 7"BP but obese, her previous partner was very lean so the BP difference between us is probably not too large). She has said previously that Im the perfect size for her since her most recent was a bit too big and her first bf was too small. But why am I hurting her? Is it because we went on for too long?