Pressure and area are inversely proportional. While you have more surface area on the circumference of the IFP, the surface area of the face of the IFP is a squared function, so the increase in surface area has a greater effect on pressure to be overcome. I'm viewing it like a floor jack (ignoring the handle). If you push the small diameter plunger down 1", you may get the large diameter jack to move up 0.1", but it will push up with 10x the force you pushed down. Since the fluid won't compress, you're basically just moving a volume of fluid from a longer, narrower cylinder into a wider, shorter cylinder. View this as the shock piston pushing fluid into the resi. The larger diameter resi will have the IFP move less for the same shock shaft piston displacement, therefore you'll get more IFP movement from a smaller resi diameter, leading to a greater ramp in nitrogen pressure.
How'd I do?