Good question actually. The length and the girth provide different sensations and press on different areas in the vagina. A long cock will get all the way up the vagina to the cervix floor. This is a very sensitive area and the sensation over there (especially with heavy pounding) is borderline pleasurable/painful. Some women just can't handle any pressure there, but even for those of us who can, too much pressure can be painful.
Now to the girth... That's what stretches the walls of the vagina. Personally, girth does a lot for me. I'm tight and pretty good with my muscles down there and just love it when I have a cock inside me that stretches me nicely. What girth also does is that when you move your cock inside and out there is more
lateral pressure on the G-Spot continuously. It's a different kind of pressure than the head of the cock. The most intense pressure I had on my G-Spot during intercourse was with a guy with a banana shape cock that isn't very thick, but just hit the g-spot in every single thrust.
Long story short, girth has a lot with creating the feeling of 'depth' and combined with a good length, you've got a winner. AND most importantly, it's how you move with your woman that can intensify a lot of the sensations.
For example, when you're all the way deep inside her, instead of moving all the time, stay in and push your whole body against hers. The feeling of being filled up is intensified that way by the external pressure of your body against her pubic bone. Some positions are also deeper than others, like doggy style, deep stick, or one leg up, etc.
My 2 pence...
