I know I didn't just read something about 'thin people are the oppressors to fat people'

no, McDonalds is their oppressor, or their overactive thyroid is their oppressor.... all of which are things that can be treated or fixed if the person cares enough to treat or fix them.
That said, no one should shame anyone for their weight. I myself am extremely thin and I don't need people telling me 'omg you're so thin' every day of my life because it gets annoying. Alternatively, I don't really care what people think about how I look and I only have a very short amount of time on this earth so I'm gonna spend it doing WHATEVER the **** I want, which I believe is a philosophy everyone should follow.. eat, drink, snort, inject whatever you want honestly as long as you're not hurting anyone else. Live your life the way you want to.