Yeesh, I should be a nutritionist, I could have told anyone that! The static 3,500 calorie per pound of fat model is still quite accurate as a baseline, but people greatly underestimate the metabolite effect. The man in the graph would follow the weight of the traditional model if his metabolism never adjusted to the additional calories.
Ever wonder why urine can be dark (almost even glowing, lol) after having one too many vitamin tablets? I believe it's because once the saturation level for a certain vitamin or nutrient is reached, the body treats it like waste... it really has nowhere else to go since it's in such excess. The same applies to calories and metabolism to a degree, the body attempts to "use" as much of the additional calories as possible to the point of saturation (over the finite period of digestion) and then converts the remainder to fat.
So he would be gaining fat at the rate of 3,500kcals/lb. the very first time he ate the cookie, and then at an exponentially slower rate every subsequent time as his metabolism adjusted to utilize the additional calories. It's also why extreme cases of obesity (400+ lbs) the person continues to consume enormous amounts of calories each day but does not gain nearly as much weight as a healthy person would from eating the same thing. Even though they hardly move, their metabolism is probably incredibly high. It would have to be to digest so much food. Sadly, in extreme cases they tend to continue eating so much that the sheer gross intake (no pun intended, lol) overtakes even the adjusted metabolism, and over a period of many years they get fatter and fatter.
The reason people trying to lose weight quickly never works is because they cut calories too much, their metabolism slows down (semi-starvation mode) to adjust. because their malnourished, the body breaks down lean tissue for nutrients. then, when they crack and cheat on their diet, the body sucks up every bit of calories in the unhealthy food and has way more than it "thinks" it needs and converts a lot of it to fat, causing the yo-yo effect. anorexics are sort of like extremely obese but in an opposite context.
relevant point here: when trying to lose or gain weight, it's just as important to consider the metabolic effect of the quantity and type of food youre eating as the net calorie loss or gain. to lose
fat one should actually eat about the same amount of calories but eat more fibery vegetables, fruit, and lean protein (bird meat rocks!) this keeps the metabolism stimulated, then you can add moderate exercise and maintain the fat burning effect of a full, healthy metabolism.
to gain fat, it's a bit more complicated but you can trick your metabolism... i've never tried this, but you may try experimenting by eating less in the morning--quick burning calories like sugar, think candy bar instead of oatmeal--to keep the metabolism slow, then very late in the day you can load up on carbs (icecream!) since your metabolism will drop even lower when you sleep, you'll hit the saturation level easily and the rest will have nowhere else to go except fat deposit. instead of eating lots of regular meals, eat irregularly to give your metabolism lots of calories to process when its not "programmed" to expect them. also, try varying your overall calorie intake day to day... try eating 4/3 or 5/3 your regular portion every 4th day or something.
anyone trying to burn fat best do the opposite of all that, program your metabolism to burn high as much as possible (and make sure eat plenty of nutritious foods to keep lean tissue intact).
Goooodluck!