I do not mean to rain on any parades here, but most of the guys who are screwing the ex have met them half way on the sexual reconciliation and this despite the fact that she (hopefully) does not have a big cock (or any cock for that matter.) Sometimes convenient sex is just that. She know you, she knows what she is getting it is just a little more comfortable that a stranger in the bar, the same way that you are fucking her for your own reasons.
I used to have one or two “fuck friends” and I have no illusions; a large cock did not hurt the relationships,and obviously they were happy with the fuck or they would have found somebody else, but like I said, there was more to even just those casual meetings than dick size
In this case he has a fairly long history with her and a son to boot. I am not saying that she does not miss your cock, but there is likely more to it than just that or she would look in the personal ads and find somebody who advertises himself as 10 inches..