It was a creative decision made the playwright who wrote the stage play it is based upon, and also wrote the screenplay. According to The Whale's wikipedia page, "Hunter stated that his reflections on teaching were the initial point of inspiration for The Whale, and that he did not add the obesity aspect until later, as a way of making the teacher have a "distance" from the audience and the other characters."

https://en.wikipedia.org/wiki/The_Whale_(play)