We are not a cuckold couple but we do enjoy mfm with a guy we know pretty regularly now, about 2 - 3 times a month. We've done it many many times with him.
We are mid 50s, friend is 35. He has a skinny nerd type look. I'm 6+, he's 8+. I will say seeing him pump her is the hottest thing ever. Seeing her wetness on his tool and him sliding in and out of her drives me wild.
The whole aspect is what's fun for all 3 of us. Watching, taking a turn, the things we say, how she reacts with him....THAT'S the fun for us.
If he had a 5 inch dick I'm sure it would be different. I don't know if the size of his dick is what makes this fun but it might be.