Most people who feel revulsion at the sight of fat people are simply a-holes. The politically correct term for what drives their ideology is seizeism: the discrimination of people based on their size. Some people, however, have an irrational fear of fat people. They have cacomorphobia.
People with this phobia often sweat or tremble at the sight of an obese person. They acknowledge they’re being judgmental, but they can’t help bursting into panic while encountering obese people. This sense that they’re bad people worsens their mental health issues. They avoid public places to not risk such encounters. This behavior can severely affect their careers and relationships.
Individuals with this phobia often trace their fears back to negative experiences with fat people. Some were attacked by obese people. As children, some were bullied by fat kids, or frightened by obese people who actually meant no harm.
There are several treatments for cacomorphobia, but only a mental health professional can diagnose someone with it.