The fact that one of your legs is slimmer than the other suggests that the muscles in that leg may be wasting away. The cause needs study. This is why your doctor has advised you to undergo tests. These tests will enable the doctor to determine the cause. In turn, this will help the doctor advise you on the means to prevent further slimming.
The tests may include examination of the spinal cord and its nerves by magnetic resonance imaging and, if necessary, electrical tests to determine whether the nerves conveying commands from the brain to the muscles in the lower limbs are functioning satisfactorily.