It is a choice. Almost nobody needs to be fat if they follow a healthy diet and do regular exercise like everyone who's slim or average does.
They're not unfounded assumptions. If you're too lazy to follow a workout programme, do some sports and follow a healthy diet, you will get fat. Simple fact of life.
It's their body, their choice. Doesn't mean I need to find them attractive for it, or that I shouldn't derive certain assumptions from it.
http://www.nhlbi.nih.gov/health/health-topics/topics/obe/causes.html
Whilst it's certainly true that genes affect your chances of becoming obese, they don't stop you from staying slim or average. Obesity runs in my family, yet neither me nor my brother are obese because we exercise 7 days a week.