The TL;DR – MacDougall wanted to prove that souls exist and his way to do so was to prove they had mass. He performed an (in)famous experiment in which he measured terminal patients at the moment of death to see if there was a loss in weight from the soul leaving the body. He later performed the same experiment on dogs to not only verify his human experiments, but also demonstrates that dogs did not have souls because their weight did not change upon death.
So, what if there was a sorcerer performing similar experiments? A guy not only cataloging the types of souls of different creatures, but also weighing soul gems filled with various types of souls to compare their weights? Could expand it further to have him speculate on what differentiates the different types of souls (intelligence, age, etc) and how a being could change the size of their soul by betterment of the self in some manner.