@ mohit
we don't need "are" because there is no ambiguity. if the sentence were ambiguous without "are", then we would need to insert it. (note that the sentence is not actually incorrect if you add "are"; you just don't have to.)
examples:
i know more about biology than chemistry
correct. (i could say "...than about chemistry", but i don't have to, since there is no ambiguity here)
i know more about shakespeare than my brother.
here, i NEED a helping word.
if i mean that i am superior to my brother in terms of shakespeare knowledge, then i have to add "does".
if i mean that i know more about shakespeare than i know about my own brother, then i have to add "about".
same deal here. there's no ambiguity (as in the first example here), so you don't need a helping word.