If you're not active enough, you do not use the energy provided by the food you eat, and the extra energy you consume is stored by the body as fat.

How does inactivity cause obesity?

Weight gain could be the result of snowball effect, new research shows, with a stint of inactivity leading to a few extra pounds, which then makes it harder to engage in physically activity.

Does laziness cause obesity?

"Researchers at UCLA have conducted experiments that suggest laziness does not beget weight gain; weight gain begets laziness. More specifically, and a bit surprisingly, junk food begets laziness." The research is published in the journal Physiology and Behavior.