Why are some teenage boys like this?

My brother and his friends were sarcastically saying that certain girls at our school were “bad”, referring to the fat ones. I thought “Damn. That's unnecessary. They aren't even pointing out potentially other conventionally unattractive girls that happen to be thinner, just the ones that are bigger.” It’s confused me. If you're a teenage guy and you think like this, share me a piece of your mind.