In a 3D bubble chart where the bubble area $A$ is proportional to a variable $z$, suppose a company has two bubble data points. Bubble 1 has radius $r_1$ representing $z_1 = 100$, and Bubble 2 has radius $r_2 = 2r_1$. If the area $A = \pi r^2$, what value $z_2$ does Bubble 2 represent?