Rob Griffiths calls this analysis “useless”, but I think there are some interesting takeaways. First, the amount of time between OS X releases began short, became fairly long with Tiger, Leopard, and Snow Leopard, and are now shorter again.
Second, as with most software, OS X has become increasingly obese. Griffiths didn’t include the fresh install sizes of Snow Leopard and prior releases, likely because it varies significantly depending on configuration, but it appears to have ballooned since Mountain Lion. (Strangely, Lion and Mountain Lion are apparently the same size, despite the latter including Retina-quality assets.)