There could be a few reasons why a person may become so obese like this.
1. Metabolism and diet... different people have different highs or lows of metabolism. I for one, have a very high metabolism... then again, I'm a more active person that enjoys going out and having fun because my body can do more things as I want to. I drink soda, which has caffeine, which gives me energy, which makes me want to go out and do anything if even for the hell of it, and in that I have about 5 cans or a 2liter or two of soda a day, and all of the foods I eat average in a week are about:
2-4 chicken soft tacos and 1-2 mexican pizzas from taco bell,
some McDonalds cheeseburgers and fries or
Jack in the Box bacon cheese burgers and curly fries.
And I go to the store and get a few things like the lunchables nachos and ovencooked pizza, along with spaggheti, top ramen soup, chocolate cereals, hot pockets and microwavable pizzas.
Went to Sizzler last Friday and got a Steak and Endless Shrimp, while my girlfriend got the Endless Salad Bar, we ate then swapped vise-versa while I had the Salad.
I also ride my bike down to the store and get a few bags of M&M's, some harvest cheddar sun chips, ice cream, and a couple of candy bars to last me for about 2 days, 3 tops.
I weigh on average 140-145 pounds and I'm about 5'9" in height, so yea I'm kinda skinny lol... its my metabolism and the energy and mood to go do things.
2. Hereditary attributes...
There are some chances of hereditary features passing down through the family, not necessarily the whole "I eat 2 much cuz my mom does", but maybe their metabolism, and their family traditions, religious beliefs, and the activity of the family.
3. Money issues
A family that has the money to afford gas and go out often to visit friends, go to the park or go out clubbing is obviously going to have a better chance of being slim and healthy.
A family that doesn't have the money for gas or a heater during the winter, and don't have many friends nearby, or live in a town that doesn't have any clubs nearby, but they can afford a TV... they'll most likely grab some food around the house like some popcorn and watch a movie... a family like this has less of a chance of being as healthy.
4. Environment issues
There are several places around the world that have really bad air.
If a family grew up not far away from the ocean, then there should be good air, and it may influence the people to go out and do something, and will definitely help them be healthier because of the good air.
However, where I live, I am in the valley of California, rated as the worst air in the world, near LA and Sacramento... which can cause people sickness and bring down their mood and motivation to do things... especially here in the Valley because of a few areas that give people "Valley Fever". I've never had it, and I hope I never get it.
The most suspected of all, the persons laziness, could be the one to blame... there are some people that live in good environments, have plenty of money to go do things, but just don't care and feel like lying around the house doing nothing but eat and watch TV... and Orion has a very clear point, that if someone really cared about themselves, they would have noticed they were getting fat, and should have done something about it, but didn't. So in this case, it IS their fault and they must suffer the consequences until they can think of what they should do. however... don't think too much, or you may start eating and get even fatter, rofl.
...damn, all this talking about food has gotton me hungry, I gotta get somethin to eat, cya.