If I assume about the equivalent of 8' of number 4AWG wire and an average of 200 Amps starter current then ---
.0002485 Ohms/foot X 8 feet X 200 amps = .3976 Volts drop
The two solenoids and airframe ground return will probably double that to about .8 Volts
That is acceptable for a 12 Volt system.
I doubt that you could get much improvement with larger wire.
The fact is we are pushing the little battery and the starter to their limits.
Maybe the lithium iron technology will solve our problem.
I built a small 115 Volt AC forced air heater for the engine and use a blanket.
That keeps the engine warm to the touch and has made starting easy on our 15/20 degree mornings so far.
Been thinking more about the hot start issue. Since I have not experienced this situation I am guessing we have a fully charged battery which cannot deliver enough amps to the starter to get best the compression stroke. This could be because the battery does not have enough capacity, the wire between the battery and the starter has too much resistance or a combination of the two.
How about installing a "gang" circuit/switch between the ignition backup battery and the main battery. When the main battery cannot get past the compression stroke in cold conditions or on a hot start, then activate the gang switch to combine both batteries to starter circuit.
Bob Anderson, CC11-00435, N94RA