Yes the size of the girl makes a HUGE difference in the appearance of the cock.
This can be proven mathematically too.
I'll use extreme examples to illustrate the point:
A girl that is 4'8" and 80 pounds on camera can theoretically appear proportionally identical to a 6'1", 160 pound girl without a frame of reference. If you don't know how large each of these women's partners are, then you still don't have a frame of reference in a porn scene.
So a guy with a 6" long penis may look very small in a scene with the 6'1" girl - in length, one would think that a 4.66" penis in the 4'8" girl would appear the same. But believe it or not, height and length is not what we are subconsciously look at in overall size, both with respect to penis and body size. It's VOLUME. And volume is measured by weight with people.
So the short girl (80 lbs) is only HALF the size of the tall girl (160 lbs) which would cause us to think that the size of the penis of the SAME guy in two scenes with each of these women is half the size (in volume) when viewing the scene with the larger woman.
Anyway I had the pleasure of witnessing this phenomenon when I dated an 82 pound girl. I enjoyed that so much there is no doubt its why I enjoy porn featuring huge penises in tiny, skinny, and/or petite women, lol.