Metabolism simply describes the chemical processes that turn your food into energy. If you have more calories in you, you're going to be using more of it up in these processes. Literally that's it.
In MOST cases, the cause for overweight/obesity is simply this:
energy expenditure > energy intake
While it's important to take into account that some people do suffer from genetic disorders or other such unfortunate occurrences, most of the time the issue can be dealt with by cutting eating and moving around more. Too many people blame their problems on myths like slow metabolism when it simply isn't true.
I know this happens rarely, but my post was outlining the main problem with leptin in overweight people. Most overweight people develop leptin resistance; only a small minority have a genetic deficiency in leptin. As such, the former is more clinically important, though I should have made it clearer in my initial post what I was trying to get at.