"The dry first stage of the Zenit masses almost 25% more than Falcon's despite carrying 20% less fuel. Zenit's second stage is carrying a 4-ton handicap relative to Falcon's, and that mass, flying all the way to second stage cutoff, comes directly off the payload capability.
Both rockets use a similar construction style, a thin aluminum skin with stiffeners welded in (Zenit's design was a major influence on Falcon 9). You would think that their mass ratios would be more similar. My guess is that improvements in structural-engineering modeling software in the 25 years between the development of the two rockets allowed SpaceX to shave a lot of weight while maintaining the necessary strength."