Look throughout history, those that relied on conquest as their ruling strategy always failed eventually and usually because of just what you described. Alexander, Napoleon, Hitler.....

The more you conquer, the more you have to hold and the troops needed to hold the conquered lands are not available to conquer more.

Plus.... conqueror types tend to be a little egotistic (understatement of all history) and the more success they have the more invincible they see themselves as. So more and more and faster and faster expansion is what they want.