Two electric bulbs marked 100 watt 220 volt

Two wires of equal length, one of aluminium and the other of copper have the same resistance. Which of the two wires is lighter? Hence explain why aluminium wires are preferred for overhead power cables. (ρAl = 2.63 x 10-8 Ω m, ρCu = 1.72 x 10–8 Ω m, Relative density of Al = 2.7, of Cu = 8.9).

We know, 

P = V2/R 

⇒More the power, lesser the resistance. 

The resistance of 200W bulb is lesser than the 100W bulb.

Also

R = ρ l/A

⇒Higher the resistance,lesser will be the value of A. Thus, thinner will be the wire. 

The wire of 200W bulb is thicker than 100W bulb, as it's resistance is lesser.

Basically, 

P ∝A