The key to preventing theft is TIME... the longer it takes for someone (...or more than 1 person) to physically detach and carry your bike outside, the better the chances he/she/they will pick some other easier target.
So having said the above, even a very heavy object chained to the bike can be lifted by X number of people and taken out of the door. On the other hand, a large, light bike rack which doesn't fit through a door will pose more of a time deterent since it will have to be dismantled before it the bike can be taken; and if the bolt/screw heads are worn away, so much the better. The best of both worlds would be to fill the hollow sections of the bike rack with something to weigh it down (i.e. sand).
Hope this helps!