never call a girl 'fat.'
even if she's not 'popular.' even if you don't like her. even if she's not your friend. even if she's not 'pretty.' even if she isn't nice to you. even if you hate her. even if she hates you. even if you love her. even if she loves you. even if she bullies you. even if she isn't a nice person.
even if she is.
don't make someone feel insecure and self conscious just because you want to make yourself feel better.