Few times that come to mind
#1 - Girl i lost my virginity to, when she first saw it her reaction said it all and she informed me later she'd actually only had sex for the first time a couple weeks prior and it was a one time thing and and he was tiny and came in seconds, myself on the other hand was so nervous we went almost an hour and i still couldn't cum, blue balls like you wouldn't believe. Anyway we fucked for a while until she wanted more, i didn't so we went our separate ways, few months later after breaking up with her boyfriend we picked up where we left off, we ended up parting ways again cause again we wanted different things, she said that was the last time but it wasn't. One night we hook up and afterwards she starts going off about how she wishes we'd never hooked up in the first place, i asked why and she says cause i was the first guy she really fucked and i happened to be the biggest which sort of ruined things from there cause she was a bit of a nympho and none of her boyfriends could really satisfy her sexually.
#2 - Old friend/FWB - We'd had a long term affair and she told me the first time how i was much bigger than her husband, even once she divorced him and started dating again we'd still hook up because i was big and a sure thing. We eventually decided to end things and she married again to a guy I've known since high school, the real ego boost being superior moment though was last year they stopped by one night to catch up and have a few drinks, it was the first time we'd really seen each other in a couple years and we were all having a good time, but we cut the night short when she says she's getting a headache and they leave. Couple days later i get a text saying she feels guilty about the other night and needs to tell someone, i was a bit confused and said it was fine that she left cause of the headache, she responds and says she didn't have a headache, she just said that cause she was insanely horny and desperately wanted my cock and needed to get out of there, she goes on to tell me they have a pretty good sex life and he is a decent length but not even close to my girth and after a few drinks that's all she could think about and she felt guilty cause she'd never even thought about another guy since she'd been with him yet a couple hours and a few beers around me and she would have fucked me right then and there if he wasn't in the room. We agreed we probably should never be alone together and we've stuck to it.
#3 - Friends GF [open relationship] we had a threesome, he is noticeably smaller. they eventually decided to try out an open relationship and me and her started hooking up on a regular basis, after a few times together i get a text the day after one of our hook ups saying how great the sex was and then she says i can't repeat what she's about to say but after being with me she kind of wished her BF was bigger, she loved him obviously but all the years they'd been together she'd never woke up sore and walking funny the next day, despite it being embarrassing and awkward, it made her want it more.