The mass of steel required to keep a big radio reflector from sagging or collapsing increases rapidly with size.
From the cnn.com
To achieve narrow beamwidths, the reflector must be much larger than the wavelength of the radio waves.
From the en.wikipedia.org
The radio waves from the feed reflect back off the secondary reflector to the dish, which forms the outgoing beam.
From the en.wikipedia.org
A Gregorian reflector system was installed in 1997, incorporating secondary and tertiary reflectors to focus radio waves at a single point.
From the en.wikipedia.org
A parabolic antenna is an antenna that uses a parabolic reflector, a curved surface with the cross-sectional shape of a parabola, to direct the radio waves.