How fat are we saying here? Fat as in John Candy (RIP) fat or fat as in "lots of people in real life" fat (i.e. ~7-10 kg overweight)? Because if the former, then it's rather obvious. Lack the overall fitness to escape the monster/dodge the blow/fight for their lives. Also often rather meek (these people are often fat from early in their lives, and therefore laughed at for years) and therefore unlikely to fight back if the villain decides to "make an example". The second option would be much less obvious, since with today's availability of food and overall sedentary lifestyle, a lot of those people would not be significantly less fit than your "optimal weight" ones. In fact, a lot of them would be fitter than average, since (A) the social stigma associated with being overweight pushes them to do more exercise (B) people who do a lot of sports when younger tend to expand in later years, since they do less exercise (still enough to keep fit, though) while retaining their eating habits from the time when they needed more calories.