Thems Fighting Words
Green Belt
It all comes down to what you define as fat. If you're talking about a morbidly obese individual who has problems walking up steps, then I'd say, run up some steps and you're safe. However if we're talking about carrying a bit of extra weight (BMI of 25 to 30), then that's different. That extra layer of fat (especially around the gut) can be highly effective against strikes and bladed weapons. About a year or so back, some guy here in Oz got attacked by a chainsaw. He survived only because of his beer-gut. So as others have said, target the legs. Those legs are already under extra pressure from the excess weight. Even if they don't fall down, it makes running away easier.