Assume that the amplitude of the EMG has a greater change from rest when the same weight is lifted overhand rather than underhand (the typical outcome in this experiment). The muscles that are doing the lifting have to produce the same force to lift the same object.
So why is the change in EMG amplitude greater when the lifting is overhand rather than underhand? Before answering this question, you may wish to assess the muscle mass at different places in your forearm.

Because of the muscle's mass that are used for each grip.

Explanation:

EMG or electromygram is a technique that is used to measure the amount of electric activity that occurs in a certain muscle during its contraction etc.

During the overhand grip, the outside of the hands are facing outwards which means that we are using the biceps more heavily but during the underhand grip, the palms are facing outwards and the triceps are used more heavily.

The strength and mass of these two muscles are generally very different and the difference in mass results in the difference of electrical activity in the muscles, causing the EMG results to be different between the underhand and overhand grips.

I hope this answer helps.