Barbie Fat is when your fat but not actually fat. Its something a model would say to another model, when she's eating a cake instead of eating a celery. Yes at that mere instance of eating just that once slice of cake you are barbie fat.
Barbie because women are often surrounded with the idea that she must look like that picture perfect barbie model and figure. The norm of looking like barbie has made me invent the idea of being barbie fat, simply being 'fat' when your not actually fat.
brunette: hey i weigh just 5 more pounds than you
Blonde: your fat
brunette: doesnt get it
me: no one will brunette, your not fat your "barbie fat"