People always judge by prejudices and projections. If they are thin, they think that being fat or huge or whatever is only matter of choice - I'm just talking about closed minded people.

Another thing is attraction, someone attracted to thin or average sized people tend to demonize something that they find unattractive - in this case - fat people. I'm different in this regard, so I know from first hand...

PS: butter is much healthier than margarine.
https://www.theguardian.com/commentisfree/2013/oct/23/butter-bad-saturated-fat-healthy-eating-industry