flat satellite antennaes- What's the theory behind them?

Dear All,
I was so used to see Parabolic dish Antennaes for satellite communications.
When I saw flat Satellite Antennaes like:
http://www.digicams-uk.com/prod244.htm
(this one is used without a feed)
and
http://www.alienworksltd.com/multi1large.html
(this one is used with feed)

They are said to be more efficient than the dish equivalents.
Why there are no flat antennaes for larger Earth Stations?
Can anybody explain the theory behind it?

Thanks in advance...