Childhood experiences, and later adult experiences, led Rick to be bulimic. He spoke of being bullied and called out at school because he was a “large” kid.
The pressure that Rick got from his friends' bullying, make him develop a really low self esteem that make him become uncomfortable in his own skin. Without proper guidance, a children at this age might think the best method to deal with their weight situation is by puking out all the food that they've eaten.