If a girl walks a lot of stairs and works out, strengthens her core and pelvis muscles, this DOES make a difference.
So a girl who wants to be tighter can actively make a difference.
That being said, MOVING YOUR HIPS and SMILING makes up for looseness.
Look at the guy as he's penetrating you SMILE WITH JOY AT HIM. In missionary, move your hips up/down, NOT side to side. Doggystyle move your hips back into him, pushing back- but not side to side.
On Cowgirl grind forward and backward with only slight lifting. It feels great just moving vigorously forward/backward on the cock - no need to porno it going up/down. Just slide forward and backwards keeping as much of the cock inside you while doing this.
With these techniques pussy/cock size don't matter - it will feel great for both.
I can't emphasize moving your hips up/down in missionary enough. When women do this it makes a guy cum very quickly, yet so few providers do it.