Print Size Issue

So I have a Prusa i3x from DIY Tech shop. I have been trying to get my printer to work for pretty much the past month. I think I am just having trouble adjusting my firmware settings.
My specific problem is this. I print out a 20mm cube, measure it, adjust settings in the firmware, print again and the cube comes out perfect. The only problem is that when I print anything smaller than this cube (or bigger) the prints come out slightly undersized. What setting should I tweak to try and fix this? I want to print out fairly accurate parts so this is quite a big problem.

Here are some of my firmware settings. If you would like to know anything else just let me know.

#define DEFAULT_AXIS_STEPS_PER_UNIT {77.364,77.741,3949.367,700}
#define DEFAULT_MAX_FEEDRATE {500, 500, 2, 25}
#define DEFAULT_MAX_ACCELERATION {2500,2500,100,10000}

#define DEFAULT_ACCELERATION 500
#define DEFAULT_RETRACT_ACCELERATION 3000

If you are sure your steps are correct..ie you attempt to move 10mm and it moves 10mm.. then you need to look for a mechanical problem. Belts tight,pullys not slipping, your not skipping steps.. do you prints look good if you print larger? Or is the surface not straight?