It's simple really.  They aren't worried about their weight.  They are just worried about the fact that other people perceived them as lazy or not wanting to lose weight.  That's why you see those fat people in gyms standing on the rails of the treadmill while the belt goes by, then posting their "miles" to instagram.    When you're earnestly trying to lose weight, it's different.  This case is more of a "I've been peer pressured into not eating pizza" effect rather than a "I want to achieve a certain body image" motivation.