I think vaginal lubrication is completely a psychological issue.
It is triggered by arousal in a woman's brain.
The lubrication is produced by 2 glands called the Bartholin's Glands, which are situated near the vaginal entrance. Therefore, a bigger cock can't "squeeze" out more lubrication.
I remember, years ago, being around the first man who ever made me really, really wet. I was on a date with him, and every time I went to the restroom, I was stunned by how drenched I was. Up to that point, I hadn't known it was possible to be that wet. And at that point, I had never seen nor touched his cock.
He did turn out to be well-hung, but not mega-hung. I think what made me wet every time I got within 50 yards of him was his personality...his body...pheromones...chemicals in the air...a vibe...who can say?
But I know some things make me wetter than others, that's for sure.
Reading a "regular" romance novel usually makes me wet. Reading an "erotic" romance novel makes me wetter. Reading a BDSM novel makes me completely, fucking, soaking wet.
But even as wet as I get from those novels, do you want to know what makes me wetter than I've ever been in my life?
My lover's words. That's right. The things he says. Not a scent he puts out. Not his cock (although it's fucking spectacular...Mmmmmmm.....) No, it's the things he does to my brain that are pure, unprecedented magic.
I didn't know my Bartholin's Glands could work this way. In just a few seconds, I can go from dry to dripping. And I would never have known that about myself, had I not met...him.
Anyway, for a lot of "size queens," since they like big cocks, it stands to reason that having contact with one arouses their brain, therefore causing more lubrication.
Whatever really turns a woman on will make her wet. If it's big cocks, well then...there you go.
Not rocket science.