Symbolism, Imagery, Allegory
Conrad was many things: brilliant, eloquent, well-traveled. He was also fat-phobic.
In the figures of Verloc, Michaelis, and Sir Ethelred, Conrad indulges his weird and recurring desire to talk about how fat people are. Fatness for Conrad has subtly different meanings in each of these men, but what it has in common among all of them is that it is a symptom of the bloated modern world.
Verloc's "fat-pig style" body is connected to his laziness (2.2), while Sir Ethelred's "double-chin" and "egg-shaped" body seem to be products of his physical and spiritual fatigue (7.8). Michaelis, on the other hand, is neither lazy nor tired, but his body has been destroyed by years of malnutrition in the prison system, and even though he eats extremely little, his body remains a big, soft marshmallow that causes him significant mobility problems.
Michaelis' obesity is a product of his victimhood and jail time, while Verloc's and Ethelred's seems to be an expression of their privilege. What obesity does in all three cases is create a general imagery of ill health and lack of vitality. This could be related to the Professors claim in Chapter Thirteen that the modern world is "mediocre, limp, without force" (13.44).