The 13" MBA air has a native resolution of 1440 x 900;
The 13" rMBP has a native resolution of 2560 x 1600, which is a pixel "doubled" 1280 x 800.
That's why the icons on the rMBP are bigger, and you can fit more information on the screen of the MBA.
You
can run the 13" rMBP at scaled resolutions of 1680 by 1050 or 1440 by 900 to display more information on the screen.
When you are at the default "Retina" setting, you are viewing the same amount of information as on a 1280 x 800 screen. But instead of a single pixel, the retina display has 4. Any element in the Mac OS optimized for this displays very smoothly. The downside is that you usually don't get any more information on the screen, it just looks smoother.
If you choose a scaled resolution of 1440 x 900 to match your MBA, the number of physical pixels vs. displayed pixels is no longer evenly distributed. You might notice some blurriness that you wouldn't at the Retina resolution. Instead of 4 physical pixels per virtual pixel, the 1440 x 900 scaled resolution would use 1.77 physical pixels per virtual pixel.
http://www.anandtech.com/show/5996/how-the-retina-display-macbook-pro-handles-scaling might help explain it a little better.