I'll bite! I have had great sex and still longed for a bigger cock. I only felt like this after I'd had a boyfriend with a big cock (where big = probably 9.5" x 7", it was a monster).
It wasn't that the sex was bad, it was just a raw, animalistic desire to be pounded by a big dick.
My current boyfriend has a big one (8" x 6.5"?) and the benefit - for me, possibly not for others - is that even if the particular sex we're having at that particular moment isn't great, I can satisfy myself in the knowledge that I'm being fucked by a fine, fine penis. I don't think this would be good enough if the sex wasn't generally awesome though: size doesn't matter that much.