The Raleigh Criterion (1.22 lambda/D) is what sets the beam angle. Lambda is the wavelength, and D is the transmitter diameter. The larger the transmitter, the tighter the beam.

Since the beam spreads as a circle and travels in a straight line, the diameter grows with distance, and beam area grows as distance squared. Total beam energy is constant at any distance till you hit the atmosphere.

The beam intensity goes as the inverse square because the area goes as the square and the total energy is constant. So that is a derived value from the physics.

For a reasonable size satellite and ground antenna, therefore you want the highest reasonable frequency and the lowest reasonable orbit.