There are extremely skinny people, very skinny people, skinny people, average people, fat people, very fat people and extremely fat people. However, all healthy newborns have roughly the same body-fat percentage.
As we grow, we gain weight. That's normal. However, the percentage of our bodies that's body-fat can and does change. I'm not going to start another pointless "is a calorie a calorie?" debate as whether it is (as I believe) or it isn't (as others believe), isn't particularly relevant.
What makes some people gain more body-fat mass & less muscle mass than others?
Where nutrients end up depends on the relative insulin sensitivity of the target tissues.
Fat cells are usually always sensitive to insulin unless they are so full of fat that they cannot accommodate any more, in which case either pre-fat cells get turned into new empty fat cells, or if there are no pre-fat cells left, the result is type 2 diabetes.
Muscle cells vary in their sensitivity to insulin. Inactivity lowers insulin sensitivity and intense exercise increases it. Body-builders do a lot of intense exercise so as to maximise muscle cell insulin sensitivity in order to get the maximum amount of nutrients into muscle cells rather than fat cells.
Liver cells vary in their sensitivity to insulin depending on how full of glycogen they are and how much visceral fat (fat around the internal organs) there is.
What makes our weight go up?
3) Putting on clothes
4) Oxygen breathed in
What makes our weight go down?
3) Taking off clothes
4) Carbon dioxide & water vapour breathed out
5) Energy losses due to movement & heat losses due to conduction, convection, radiation & evaporation
6) Miscellaneous (loss of various bodily fluids, loss of skin cells/hairs/nails, ketones in urine/sweat/breath)
Some factors are controllable/reversible and some aren't. Over a period of 24 hours, our weight goes up and down by a few pounds due to the above factors. Whether our average weight over a 24 hour period changes over the course of days, weeks, months & years depends on the balance between the things that make it go up and the things that make it go down.
Why do we eat & drink what (and as much as) we do?
When we are young, what & how much we eat is determined by our parents (also schools). They dictate the foods and the portion sizes. Poor parents (also schools) often buy the cheapest possible foods. Poor parents encourage "plate-clearing" as they cannot afford waste.
Some of our ancestors lived in hot countries and some lived in cold countries. Some habitually ate meats and some habitually ate shoots or roots or fruits or grains. This has an effect on our bodies. My ancestors came mostly from Northern Europe which may explain why I achieve better appetite control on a meat-based diet rather than a grain-based diet. The ability to digest lactose (milk sugar) is determined by the habitual milk-drinking in adulthood of our ancestors. Only 4.7% of white English people are lactose-intolerant compared to ~98% of Africans, who would have drunk warm raw milk that had lactase in it.
3) Peer pressure from parents, siblings, friends, business partners & significant others
"Go on! One (more) *insert name of junk food/drink here* won't hurt!"
It's become commonplace for English people to stuff themselves silly at Christmas, eat lots of chocolate eggs at Easter, pancakes etc.
Certain foods that are very nutritious are either culturally-unacceptable or have fallen out of favour e.g. rabbit/horse/cat/dog-meats & offal (brains, stomachs, lungs, pancreases, hearts, kidneys, bladders, necks, feet).
Increasingly busy lives make some people buy pre-prepared meals/snacks which are usually refined carbohydrate-based e.g sandwiches, Subway/Maccy D/BK/KFC. Some workers only have access to vending machine foods & drinks or canteen food which may be of dubious quality. Others blow-out on business lunches.
How many people eat by the clock rather than when they are hungry? School children & many workers have no choice and have to eat at set meal times.
There are lots of cookery programmes with celebrity chefs endorsing some diet or other and TV adverts for all sorts of manufactured foods but not many adverts for meat, poultry, fish, eggs, cheese etc (whatever happened to "Beefy & Lamby" & "Go to work on an egg"?). There's always some "expert" telling us what to eat & what not to eat. A lot of mindless eating occurs while watching TV.
9) Physiological & psychological reasons
When we're feeling ill, sad or depressed or have unstable blood glucose levels, we may fancy foods which are high in sugar and fat (mmm, chocolate!). People who are very sedentary and/or lacking sufficient Vitamin D may have unstable blood glucose & insulin levels resulting in extreme lethargy after meals followed by ravenous hunger. People with Anorexia Nervosa often starve themselves or purge after meals.
10) Allergies & intolerances
People avoid foods that make them feel ill.
If we live in a country that grows a lot of a certain foodstuff e.g. rice, wheat, beetroot etc, we are encouraged to eat a lot of that particular foodstuff. When we feel hot, our appetites decrease and when we feel cold, our appetites increase. This is why we don't get fat when we put more clothes on to make ourselves feel warmer.
This isn't so relevant, now that most foods are transported around the world and sold in supermarkets, but locally-grown seasonal foods bought from farmers' markets are tasty & nutritious.
The saying "the Devil makes work for idle hands" applies to our brains & stomachs as well.
Some people's appetites decrease when they exercise and some increase. I used to fall into the latter category. Over-training at high-intensity on insufficient carbohydrate intake can drain muscle glycogen to the point where muscles rapidly suck glucose from the blood causing low blood glucose. Apart from faints, shakes & sweats, this hugely increases appetite as the brain is crying out for something to raise blood glucose a.s.a.p.
Lacto-ovo-vegetarians, pescatarians, vegetarians, vegans etc will not eat certain foods for ethical/moral reasons.
The sight & smell of food & the sound of food cooking can increase our appetites. TV adverts and supermarkets use this to increase sales.
The emptier the stomach is, the more ghrelin it secretes, which increases our appetites.
If clothing becomes uncomfortably tight around the waist, that can suppress our appetites. Keep your belt on the same hole, to discourage over-eating.
19) Self Shaming
If we catch sight of our bodies in a mirror and don't like what we see, that can suppress our appetites. People who have Anorexia Nervosa see their bodies as fat/obese when they are actually skinny/emaciated.
20) Societal Shaming
In Japan, it's frowned upon to be too fat. Ditto in "Rich" areas of some countries. Fat-shaming can suppress appetite.
21) Current fatness
As we get fatter, fat cells secrete more leptin, which suppresses our appetites. Very fat people's fat cells secrete so much leptin that the brain can become insensitive to it, resulting in poor appetite suppression.
Some people find it harder than others to resist the enticements listed above to eat/drink more calorie-dense, nutrient-poor junk.
If I've left anything off this list, feel free to comment. Our bodies are pretty complicated and contain many regulatory feedback loops, so we humans have managed to survive famines & disasters over the aeons by our ability to store an excess of proteins, carbohydrates & fats as muscle & body-fat (also food in food-stores) and are now at the top of the food chain (except in lion, tiger, wolf, hyena, bear & shark territory!). Our biggest threat today is excessively-cheap & over-promoted manufactured foods which are calorie-dense & moreish and lifestyles that encourage us to over-eat, under-move and under-sun our skins. When people get too fat, their blood glucose control becomes impaired, which encourages even more over-eating and under-moving, thus creating a vicious circle.
I.M.O, manufactured foods should be taxed and the revenue used to subsidise natural foods. One problem with such a plan is that the Government doesn't always use revenue for the purpose intended e.g. Road Tax. Another problem is in defining manufactured foods e.g. does churning milk to make butter count as manufacturing? Ditto pressing olives to make EVOO? I personally think not, but it's a grey area.
I also think that there should be a ban on the advertising of manufactured foods, as adverts encourage us to buy & consume foods we don't need. Marketing is more persuasive than you think.
See also Eat Less, Move More: Solutions to problems.