Can there be any doubt that most (not all) guys will exaggerate their sizes to women? Do you think most do? How many guys have lied to you and you knew better but didn't say anything about it? My guess would be that if he says he's 6" he's probably 5.5" or quite possibly even less. If he says he's 7", he's probably no more than 6.5" or quite possibly even less.
Maybe much less.
Maybe much less.