(non-Anonymous MD chips in)
That's why you can have someone who's had their stomach stapled and can't eat more than a plate's worth of food a day get fat.
They don't get fat. They were fat in the first place, if they had their stomach stapled. I've seen people who worked industriously to overcome their stomach stapling surgery to fit in as many calories as possible ; making sure they consumed only the lowest-bulk, highest-calorie foodstuffs, and wonder why they didn't lose weight. I mean, they had the surgery, and that's a miracle golden ticket to weight loss, right???
Also note that fat isn't just made of food. The air you breath and the water you take in also adds to the chemical process.
Mhhmmm, but it's the same air and water pretty much everywhere. Unless you live in a cotton candy cloud next to the gravy pond, it's not a factor in determining your weight relative to the next guy.
The two overwhelming factors that govern weight are....
* Dietary habit. Not just how much you eat, but what. Because "what" has a serious impact on "how much" - like those stomach stapler guys, it's much easier to eat too many calories if you ingest it in the form of low-bulk, highly processed foods. Yes, if you choose your car based on whether it has a beverage holder which will take a Big Gulp, you're one of these people. You can eat huge plates of vegetables and not gain weight, because they are mostly composed of that water you're talking about, and you can make them tasty with herbs and spices and ... canned tomatoes, makes any plate of veggies 100% more interesting. The other important habit is your shopping habit - just not buying those low-bulk calorie-dense foods and not having them around is very effective.
* Excercise habit. The simplest being to walk and not drive. This is why America is so far ahead in the fat stakes compared to Europe - many things are too far apart from each other to walk, in contrast to Europe which is a little more compressed. I visited Oregon and people looked at me funny because I annouced I was going to walk to places as far away as half a mile or so. Where do you have the least obesity? Places like New York, where everything is in practical walking distance. Once you get fat, it's like a trap - everything feels like too much effort to do, so you do less and less. Your knees end up too damaged to walk or gasp run.
All this is from experience (although I've not been what I'd call "fat" in a long time, I'll raise my hand to being overweight). My marriage ended last year, and feeling the need to make myself a little more attractive for the dating game, I dropped over 20 pounds in 6 months from cooking for myself instead of buying pre-made food, and getting off my butt and going for a run once or twice a week.
I completely get that people find this hard, because I do. For most people, weight is a psychological issue, beause as a species we're hardwired to get as much food as we can, so to maintain a proper diet we have to use our front brain instead of our lizard brain. But the excuses like "it's my metabolism" or "it's the water" do nothing to improve the condition, they're just the mental equivalent of more junk food - something that makes you feel better about the problem but gets in the way of resolving it. Hence the emphasis I place on the word "habit" - making decisions is tiring, but if it's just "what you do"... then not so much. Cooking decent healthy meals for my daughter twice a week (with enough for leftovers to keep me eating well the rest of the week), and hopping on the rower for a minimum of 10 minutes a day, is now "what I do".