By sitting on the chair most of the time during work my buttocks are growing bigger.
Is there any warm up to reduce this effect?
Your butt is not getting bigger because you are sitting. It grows because you are probably eating too much and it accumulates fat.
Change your diet, as your profile says you are from India, this answer might help you:
If you have a typical Indian diet like I used to (though I don't want to typecast or stereotype here), it's probably very carbohydrate-heavy. This means a lot of wheat, rice, potatoes and sugar.
Check your drinks as well. Soda and Juice are high in calories, too. Switch to drinking water.
Exercising might help you lose weight, but a healthy and balanced diet is the most important part in achieving your goal. Don't try to invent excuses for your body fat to increase when there is a simpler explanation.
I agree with Baarn. The reason your butt grows is probably because that is where your body is likely to store fat. It's different for every person.
Even if you think you don't eat much it might be a lot for the energy you are using during the day. I will tell you a story - I have a friend who doesn't go out much and, like you, spends a lot of time sitting. At one point he asked to lose some weight so we spoke about a diet. However a normal diet didn't work for him, it turned out he had to cut DRASTICALLY on carbs in order to start losing weight. Which was unhealthy.
Thus what you should aim for is increase your physical activity - if you don't want to exercise then simply walk. Walk for at least 30 minutes everyday. The more - the better. Look at the situation this way - when you eat you consume calories. When you perform during the day you burn calories. If you burn less than you consume whatever calories are left will be stored as fat in your body(your butt in your case).
So you can control and limit the amount of calories you consume but you should also increase the amount of calories you burn.