Why does the optic need the same diameter? For gathering all of the light, with a mirror at least, I think you could place a smaller optic closer to the light and still capture the same fraction of the light?
I probably am missing something about optics though.
The ability of an optical system (a lens, mirror, or combination) to collimate the output of a light source into a narrowly diverging beam is directly proportional to the diameter of the optical system's final element and inversely proportional to the size of the light source.
Bigger optics = less beam divergence = more useful range.
This is a function of the diffraction limit[0] and is a property of physics.
If you want to match the beam divergence of a 30" optical system, you have to use 30" optics; there are no shortcuts.
I know that we can create very large virtual apertures for receiving signals by using arrays of many receivers. The basic diffraction physics presume a single source, so it isn't totally obvious that they generalize to a multi-source system.
I can, however, imagine that a multi source system would require in-phase light sources to kind of mimic a phased-array receiver on the send side.
I probably am missing something about optics though.