In the case of fat girls, their attempts to copy the good times of skinny girls are intepreted as "desperate" and hilariously sad.
Evidence of fat girls having fun can often be found in facebook or myspace photoalbums taken in front of bathroom mirrors, before high school dances, or by apple photobooth. Worst of course, are the photos taken by the beach.
Throwing up gang signs or making seductive kissy faces are quite common.
Girl 2: "I did. Can you say "fat girls having fun?"