I don't recommend having this argument. It's useless in almost every respect.

There are two fundamental issues. First, most people don't understand what a Calorie looks like, and think the difference between a healthy weight and an unhealthy weight is a large amount of food, rather than a small amount of food compounded over long periods of time. Want to lose weight in a sustained and sustainable fashion? Subtract a small amount of food over a long period of time. Instead, people crash-diet, then go back to normal eating habits.

An extra apple a day translates, over years, to up to 50 extra pounds. Looking at two people's daily diets, one is overweight, one is healthy, and most people couldn't tell the difference by looking at what they ate.

The second problem is that exercise is incredibly unpleasant if you're overweight. If you're currently in shape, try tossing 50 lbs of weights into a backpack the next time you exercise. Or better yet, don't, because you could hurt yourself pretty easily in exactly the ways overweight people injure themselves when doing things like jogging.

It takes physiological issues to gain serious amounts of weight in the first place; these won't stop you from losing weight, but they'll make it harder to maintain a steady weight. Normal people fidget or otherwise increase their base level of activity when they overeat, burning off excess calories. Overweight people have to be more deliberate and conscious of these things.