My guess is that if you put a blindfold on her and was vaginally penetrated by the two of you, yes she could easily tell the difference. But we're not talking about night and day here. He obviously isn't Shane Diesel. My guess is that there's a whole lot more concern from her about the guy attatched to the dick, if there are any serious comparisons going on. Besides, although I'm a little bit larger than your girlfriend's ex, I've been told LOTS OF TIMES that less "girthy" guys aren't quite as taxing when it comes to anal and oral sex. Stop worrying and put your more than adequate cock to work pleasuring the girl.