If the ailerons are moving twice as far one way than the other, then either the airplane is rigged incorrectly or the radio has something set incorrectly.

If the ailerons are on individual servos, and are moving the distances you described (both up twice as much as both down) and your rigging is "square", then it sounds as if one of the special functions is working to inhibit the downward movement.

If I were you, I'd post your problem in the special support forum for JR radios.