Is sitting on your butt making it flatter?

Probably not, but if you don't get regular exercise, you can gain weight in your gluteal region and it might appear to spread out. Also, if you sit alot in your job, make sure you are getting proper exercise and nutrition, hydration so you do not develop hemmorhoids. It is very common for people who sit for long periods to develop hemmorhoids due to the pressure on the colon, and the blood vessels. Good luck with that :)