wavelength and L/C ratio
As the link says, it's a design optimisation. They want to maximise the Q-factor (energy retention versus power loss) of the circuit, but as your wavelength increases (and antenna), the amount of stray capacitance increases. They use 1.5pC*L as an engineering rule of thumb to get you the amount impedance you need.

Read more on Q-factor here to understand it in the frequency spectrum: http://en.wikipedia.org/wiki/Q_factor