eHow.com is one of my favorite websites. It gives me so much random information about so many different things that I enjoy reading a lot from it. I learned that it takes approximately 3500 kcal to lose a pound (that is, 'calories used' subtracted from 'calories taken in' must equal 3500 kcal). Now understanding that, we can see two main reasons for individuals gaining weight.
Reason number one is that some people take in more calories than they use. Individuals take in a massive amount of energy so that they come to a point where they continue to add to what they do. Because the body uses more energy when it gains weight, the body will continue to grow in weight until it reaches a balance where the energy consumed equals how much the body needs to use (and until then, the body will continue to gain weight to reach this homeostasis).
The second reason is basically the same as the first reason, but considered from the opposite perspective. The second reason is this. People use less calories than they use. The focus of this reason is on the calorie usage. Sometimes, people eat (what should be) the proper amounts for their body, but due to their lack of physical activity, they don't use what they put in and their body stores the excess energy. Thus, the body does the same thing reaching the proper balance.
Homeostasis is when you are in a stable state, when depending upon different elements. The two major elements in this equation is energy consumed and energy used. The body will reach a stable state (balanced state) in which the energy consumed WILL equal the amount used. So why talk about standing vs sitting?
Going back to eHow.com, I figured out that there is a 12-30 kcal of difference between sitting and standing (per hour) which means that should the only thing you change in your day is the amount you sit/stand, if you lose a pound between 116 hours and 40 minutes to 291 hours and 40 minutes of standing. On average, if you replace 204 hours and 10 minutes of sitting with standing, you can lose a pound. On the other hand, if you replace the standing with sitting, you will gain a pound. Obviously, you won't make that huge difference in one day, but over a longer period of time, your body will add it up (like a bank account) and what you put in will show.
It usually won't take one day to make someone a millionaire, and in the same way, it won't take one day for someone to gain or lose weight. Instead, it takes multiple days... weeks... months, for that kind of change to happen. If you want to make a big difference, it'll take a longer amount of time, but fighting your battles one day at a time, you can end up making a big difference in the long run.