I had a FWB for around 2 years, I’ve never cared about or understood the woman has had a bigger dick problem, even with my wife, but I think maybe you have more feelings for this FWB than you think, and I also have a story about size, probably your FWB is being honest about not enjoying her experience.
My FWB called one day and wanted to talk, so we had dinner. We had both been tested so we could enjoy unprotected sex (I’m fixed), and had agreed to disclose any sex, especially unprotected sex that occurred with others for our safety.
She told me her and her girlfriend had been drunk at a house party, and she had fucked a young guy who her girlfriend had told her had a huge dick in the bathroom.
She said halfway through she sobered up because of the pain, and also realized what she was doing, got off him and apologized. He asked her to at least suck him off, but she said no, he got an attitude naturally enough, so her and her friend left the party.
I asked her just how big “huge” was, she said he was about as long as her forearm, (5’2” girl), so I’m thinking around 9” NBP. She said she would never consider one that big again, so your FWB is probably very happy with you, you need to get your mind right and enjoy the sex.