There was one study showing that too much sitting down could make you more likely to gain weight. The more you sit on your butt, the less active it would be. An inactive butt is a weak butt, and a weak butt leads to bad posture and weight gain.
So, we're not just asking people to just get off their butts to exercise. We want people to stop sitting on their butts too much.
Your butt muscles (gluteus maximus, gluteus medius, and gluteus minimus) make up the LARGEST muscle group in your body. This means that they need more calories than other muscle groups. Or simply put, it's the butt muscles that burn the most fat AS LONG as they are strong and active. This is why exercises like squats and lunges burn more calories than perhaps any other strength-training exercises -- all because these exercises involves the butt muscles.
Yet, when you sit on your butt all day and do it often, then your butt muscles could literally flatten out. Is your butt flat? Have you seen others with pancake booties? When your butt is inactive, it shrinks and could get flat -- and this is a condition known as atrophy. When your butt atrophies, it requires less calories to function -- or in other words, it burns LESS calories, which ultimately means that it burns less FAT.
Who would have known? The smaller your butt, the more weight you'll gain!
Think about it. How many overweight mail carriers (UPS, FedFex, etc) do you see overweight? Barely any! How many customer service, officer workers, phone operators, etc., do you see overweight? Plenty! They are on their butts all day! Mail carriers aren't.