The process of enlisting more and larger motor units to produce a stronger contraction is called:

The process is called - Motor unit recruitment.  

Explanation:

Motor unit recruitment

Motor unit recruitment  helps to increase the contractile strength of the muscle by activation of the additional motor units.

A motor unit consists of motor neuron and all the skeletal muscle fibers that are innervated by axon terminal of the motor neuron. The collection of motor units work together to coordinate contractions of single muscle

Now, when a motor neuron gets activated , all the muscle fibers gets innervated by the motor neuron and stimulated and contracts , this activation of the motor neuron will cause the weak muscle contraction.

As a result , activation of more number of motor neurons will cause more activation of muscle fibers and hence, lead to stronger muscle contraction.