I've had long term girlfriends and gotten close with their families. I've seen a couple of their dads. One would take me to the pool me, him, and a gf's brother. I hung bigger than both of them. The brother nodded and said "Okay bro" with a bit of a laugh. The dad looked at me a bit longer than a glance when his son made the comment when we first undressed and then glanced over a few more times.
Another time a gf's dad and I were hanging out drinking on the deck one night and I went to piss off the side and he said "Somebody has a bit extra in the pants." Then he said if we both have to piss at the same time, he will pick the opposite side for privacy and so I can't see his shortcoming.
Another instance, a gf's dad and I shared a hotel room for a wedding, while my gf and her mom shared a room, because her parents didn't want us sharing a room even though we were adults and had been dating. I hung out in the room in underwear and noticed he took some glances. The next morning he was in the shower and I walked in to take a piss. Instead of talking to me through the shower door, he slid it open to look at me as I pissed and I saw him looking at my dick mostly, instead of my eyes.
I saw his through the shower glass and his was a decent size, but smaller than mine. I then laid on the bed and he came in the room to finish dressing and out of nowhere he says, "Now I see why you fill them (meaning my underwear) out." I did a half laugh and texted my gf let's get breakfast, so then I could escape the room fast.