This has happened to me a few times.
The first time was my straight friend from college. He’s seen me naked pretty often because I always strip down when I’m drunk. Then one night he was wasted and spilled beer all over himself. He was always super modest, so we were both shocked when he dropped his pants and underwear. He always told me mine was bigger than his, but I was still surprised to only see about 1.5”. We never talk about it and it really doesn’t even cross my mind when we hang out.
The second time was a gay coworker. He was a bit cocky at times so we didn’t really hit it off, but he moved in with a mutual friend, so we all ended up hanging out one night. When our friend went bed, I got up to leave and he invited me to his room. He was already hard, a little less than average length (5”), but the thinnest dick I’ve ever seen. It felt like I was holding his finger, but something about the size difference turned me on. I think this is when I started fantasizing about fucking guys with tiny dicks.
The last was a childhood friend’s older brother. We caught up after seeing each other at a local gym and decided to work out together occasionally. After our first workout we went to the showers and I took the stall across from him. The wide gap between the wall and glass doors made it hard not to see or be seen, so it was pretty obvious when he started jerking. I couldn’t see the head past his fist, but when he let go, it looked like he was packing maybe 4”. I feel a bit sorry for him because I’ve seen his younger brother naked and he’s almost twice as long.