I got measured once, in the company of her long term lover, quite early on. I had been asking her "how come I didn't make her orgasm, and he did?"
She said it was "because he's got a bigger dick". I'd seen it and didn't think it was that much bigger.
So, when we were next all together, she got us both naked and erect and measured us.
I was 5 and a quarter long (or 133mm), and 5 around (128mm). He was 6 and a half long (165mm) and 6 and a half round (165mm).
I suggested that there wasn't a big difference in size.
She then calculated the volume. I was 10.4 cubic inches (173 cc).
He was 21.9 cubic inches (359 cc).
He was more than twice my volume!
My wife said "therefore he's twice as big as you, which is probably why he makes me orgasm, and you don't".
They spent the night in our bedroom, while I got the couch, and her wet panties, which she made me put in front of him before they left.