Hypothetically, these two neurons (different sizes) will have the same resting potential and will receive an identical stimulus.
The small motor neurons
has a smaller surface area so less ion channels available and therefore a higher resistance to change (it also has less cytoplasm which increases resistance further)
Ohms law(V=IR) states that a higher resistance (same current) will produce a higher potential
therefore the smaller motor neuron will reach threshold before the larger one
The smaller cell has increased resistance, excitability, EPSP amplitudes, post-synaptic delay, duration of after-hyperpolarisation…. et etc primarily because of this feature (less ion channels and less cytoplasm)