The idea is that they may  think or claim to be healthy (cough tumblr cough) but n reality their perception is extremely skewed, and they actually have been so fat for so long that they have been conditioned into believing that unhealthy things are healthy and vice versa. Like, they might think their morning "jog" around the block is good, but in reality its not enough. They may think its enough to not eat sweets one day a week, but in reality its way more than that.