Become a MacRumors Supporter for $50/year with no ads, ability to filter front page stories, and private forums.
It's quite normal for it to get hot, especially under heavy workload. If it's too uncomfortable, set it on something. Just make sure you don't set it on a blanket or pillow or anything that can block the air vent at the rear, near the hinge.
 
I usually use it in bed. It sometimes gets really hot (80c) so I prop it on a cushion. It's not heavy, though.
 
Register on MacRumors! This sidebar will go away, and you'll see fewer ads.