In the Pokedex's Name Origin, Zekrom's name originates from "Ze" meaning a stronger character then "Re" and on Reshiram's, it says "Re"! So is Zekrom more powerful?
Yin and Yang are forever equal.
He is not actually weaker stat-wise(both have 680 base stats). It is referring to the fact that Zekrom was more aggressive than Reshiram.