Changing facet_grid label sizes

This does not change the facet grid label font size:

library(ggplot2)
ggplot(mpg, aes(displ, cty)) + geom_point() + 
  facet_grid(cols = vars(drv)) +
  theme(strip.text = element_text(size = rel(2))) +
  theme_bw()

But this does:

library(ggplot2)
ggplot(mpg, aes(displ, cty)) + geom_point() + 
  facet_grid(cols = vars(drv)) +
  theme_bw() +
  theme(strip.text = element_text(size = rel(2)))