Question

Why is the door described as being heavy

Answer 1

Because the door is heavy.

Answer 2

The most ancient doors were made of timber, such as those referred to in the Biblical depiction of King Solomon's temple being in olive wood.

So it's mainly because of the material and how big it is. Since many doors are bigger than we are that serves up a purpose. There's your answer.

From centuries ago the doors we made now did not exsist, it was made from a completely different material. It used to be called fire wood to block fires.