I'd say 100% of the time. Some of the reactions are truly memorable. But everyone I have had sex with the first time had something to say about the size of my cock, some expression of disbelief more often than not.
Every time.. The most memorable are often where her own "monster horse cocked" husband or boyfriend or lover is standing there and she...usually both of them...say something.
This experience happened a good time ago...back when I was an active member of AFF...Adult Friend Finder. Long story short...two women, friends, who were both having sex with the same hugely endowed guy who was on AFF, decided they wanted to have a sex party with anyone else who was huge in the area. They were from Orlando, Florida....so they found and contacted four other guys (I was the only one who was single...who was not in a couple or married relationship), and worked out the details for a sex party over a long weekend in Orlando. A guy and his wife who were from NJ but also had a home on the south Gold Coast near Fort Lauderdale, they drove over, a guy and his two "wives" from over in the St Pete area, another guy and his wife from the panhandle, and myself. The woman in the couple from the panhandle area, she was a sort of thicker but not fat MILF...easily the biggest boobs of any of the six women there, 38H's at least....when everyone got nude and all five of us men got erect...said the line that the other women all agreed was true. She said, as I recall "This is incredible....I can't believe the size of all these giant cocks...I always thought my husband had the biggest cock in the world...I can't believe this...incredible". As it turned out...when the women compared and measured us all, her husband's extremely long cock tied for length with the black guy from the Fort Lauderdale area as the "shortest" of the five of us, and was fifth in girth...but the other five women, except the wife for the guy from Fort Lauderdale, all agree with her..they all thought their "man' would have the "biggest" at the party and were surprised that it didn't;t happen that way...the woman from Fort Lauderdale area...she was surprised her husband wasn't the biggest there...but she said she knew he wasn't "the biggest in the world" as she stated several times she thought their son had that "title". A year or so later...turns out he was enormous...but she was very upset that her humongous endowed son, at another long weekend party, he "lost" that comparison too. That really upset her...but that was another experience...the point is that like the poster...everyone of those women, thought going in that their own "monster cocked" husband/boyfriend was going to be the "biggest" and all were surprised to see others who were similar and even longer or thicker or both.