Why were Montag's colleagues “suddenly fat and odious in their plump fireproof slickers"?

This quote comes from Part 1 of Fahrenheit 451 and follows the disappearance of Clarisse McClellan. Clarisse, Montag’s neighbor, was an unusual girl who did not act like anyone else her age. Clarisse is the catalyst for changing Montag’s view on his life and profession. She asked interesting questions and inspired Montag to wonder about the character of the world he lived in, rather than blindly accepting it. This is evident in the conversation he has in the fire station at the card table, when he asks what happened to the man whose library they burned in the previous week. Beatty answers him:

“They took him screaming off to the asylum.”
“He wasn’t insane.”
Beatty arranged his cards quietly. “Any man’s insane who thinks he can fool the Government and us.”

Montag asks several other questions in the course of this conversation which requires Beatty to remind him of the "history" of firefighting, illustrating Montag's dissatisfaction with what he thought he already knew. Montag’s distraction is further illustrated when he nearly forgets his helmet at their next call, which happens shortly after their game. As they arrive at a woman’s house to burn her library, Montag sees his coworkers as simultaneously ridiculous and dangerous agents, rather than heroic men, fulfilling a duty to their society. He is aware that he looks the same.

In Ray Bradbury’s dystopian novel Fahrenheit 451, protagonist Guy Montag starts to question the way his society operates. A young girl, Clarisse McClellan, asks Montag questions that spark his own line of questioning. Montag’s questioning attitude begins to change the way he sees the world. He wonders about the firemen that used to put out fires and how firemen now burn books. He starts to see that there are things wrong with the way his society works, and this changes his perspective. He no longer sees his fellow firemen as good guys. This is why Beatty, Stoneman, and Black are “suddenly odious and fat in the plump fireproof slickers.” He sees them as despicable and ridiculous in appearance, because it matches how he views their actions.