The gain formula is approximately
Gain = 17.5 + 20 * LOG( F * D)
where F is the
frequency in GHz and D is the diameter in metres.
Therefore as F increases for a given dish diameter,
the gain increases. You might think then that if
you choose a 4 feet (1.2m) dish size, you just get
more gain at Ku gain, and that's good right?
Well, no, because the consequence of greater gain
is narrower beamwidth. At some point the dish pointing
accuracy requirements are too severe, in other words
the antenna beamwidth is too narrow. Slight pointing
errors will result in flutter of the signal, for
example due to wind vibration.
service has a given almost constant gain requirement,
because the link parameters are stable with time,
varying only by a few dB due to rain or due to sun
noise. You therefore need just enough gain to satisfy
the link loss budget, and having more gain achieves
nothing terms of system performance. It just costs
more bucks, and gives more alignment headaches.