I had a different model in mind. I think there's a satiation point on exposure. If you were raised by consummately literate native English speakers, then perhaps you acquired a choate vocabulary simply by immersion in that environment. Additional reading of fiction therefore, will offer you only the meagerest of marginal improvements. Many people weren't raised by excellent speakers of English though, and these individuals are the ones more likely to gain from increased exposure.

Would you make the same statement about the relationship between reading novels in Spanish or German and improving vocabulary in those languages?