Being fat is a choice the vast majority of the time, and I have a huge bias against big people.
I used to be fat (250ish lbs (110ish kg) at 5'8"ish (172ish cm)), and as much as I would like to blame my shit on anything else, the person feeding me, the person sitting at the computer for hours, the person actively avoiding all physical activity was me and no one else. After I got diagnosed with some weight related shit, I turned my entire life upside down, am at a much healthier 150 lbs (68ish kg), and feel so much better, both physically and mentally.
I'm aware of my bias, and I make every active effort to counter it in my actual dealings with bigger people. Especially because there are certain circumstances, however rarely, where it may not actually be their fault. But I'd be lying if I said my initial impression was anything except "God, what a lazy, fat fuck."