I'd like to talk about cars for a bit.
A 2006 Lamborghini Murcielago weighs about 3700 pounds and gets about 9 mpg in the city (13 on the highway).
A 2008 Saturn Aura XR Sedan weighs about 3700 pounds and gets 17mpg city, and 26 mpg on the highway.
Now, brand new, that is a significant difference in fuel economy. The Lamborghini requires TWICE as much fuel to move the same amount of weight the same distance as the Saturn. As the cars get older, parts wear down, degrading fuel economy further.
While the vehicles have some differences, the basic idea behind each is the same: An internal combustion engine with all the usual stuff: Muffler, radiator, that sort of thing.
Most people generally accept that, while they don't necessarily know all of the science behind what makes a car go vroom vroom, the minor differences in design are all that's necessary to make two vehicles of the same weight have vastly different fuel needs.
These same people, however, cannot seem to accept that one person's body uses fewer calories to perform a task than another person's body does. Even though our bodies are infinitely more complex and variable than automobiles, the public consciousness seems to be permanently mired in the notion that everyone's body should be the same as everyone else's.
Well, they're not. Some bodies are better designed for endurance, while others are more agile, better able to have children, stronger, more able to withstand a disease or famine, or any of a myriad of variations that have cropped up in response to environmental stresses. If you are wealthy enough, you may be able to use medical technology to make your body do what you want, just as you have more power to choose what car you want. Most of us, though, have to live our lives with the bodies we have, be they fat, beautiful, strong, short, long-lived, or chronically ill.
Some of our bodies are very efficient at using and storing food energy, probably indicating that our ancestors needed this trait to withstand an environmental pressure. Other bodies are like sports cars: They burn energy like a wildfire consumes Southern California mansions, indicating that they are designed to have a relatively constant, dense source of energy. Each type of body shows wear and tear differently, just as different types of vehicles wear their parts out differently. Different, however, does not mean that one is better or worse than the other; it just means different.
This is just a prelude to some other points I will be making later on. I would like to at least point out, though, that being poor is likely a great deal harder if you've got the less efficient body (or car, for that matter). Partly because of this, I grow more disgusted each time I read some columnist whining about how fat poor people are. I suppose they'd rather see these people starving and miserable; after all, what's the point of being a middle-to-upper-class talking head, if you don't have those poor saps to look down upon, right? And if you can claim that their being fat is the result of ignorance and sloth instead of a natural selection for energy efficiency, then, gosh, you've just absolved yourself of having to care about poverty.
I'll stop there for now. I'm not done with the "OMGLAWSOFPHYSICS" yet, though. Stay tuned!