The doors are that heavy for a few reasons.

First is the physical size. Since the car is a coupe that doesn't have rear doors, they are longer than average doors and have longer glass and longer window regulators.

Second is for safety reasons. There's a big bar running down the middle of the door that is there to prevent the door from caving in on you the moment some dumb soccer mom decides to forget that she's driving with other traffic.

Erik P.
Official 2GNT thread Hijacker
Some say that he's driven over more Covenant than he's shot, and that his grenades aren't where you'd expect them to be...
All we know is, He's not The Stig... But he is The Stig's Spartan cousin!