It is the main bulk of the phone and also the most power hungry part of the phone. When you unplug your charger after charging go to the battery status info page and you will probably see it is using 90%+! but over time (as you have more uptime and therefore more data) it will settle down.
I wouldn't worry about it, it's just obvious that a screen that big is going to take some power. So if you are constantly playing games, surfing the net, on facebook etc the screen is always on and in use.
I wonder what the figures would say if you left it on standby in a draw all day checked the info at the end of the day.
Paul.