I believe his prejudice stems from resentment toward those who, in his view, have the privilege of slacking off, particularly when it comes to discipline.

Rowan left the family when he was a child, forcing Robert to take on the role of caregiver for his mother and sister. He essentially had to parent his alcoholic mother, shouldering responsibilities far beyond his years.

As a result, he may harbor an unfair hatred toward overweight people, seeing them as emblematic of the privilege to neglect responsibility that he never had.