To put it simply, lower voltages require thicker wires to achieve the same amount of current, copper of a said density and length can only transfer a said amount of amperage due to resistance, if you go too far it will heat/degrade the cable which will also cause voltage drop.
Higher voltage = less amperage needed for the same amount of current.
Lower voltage = more amperage needed for the same amount of current.
Here's a nice example you might be able to follow, hope it helps.
A 10ft 28AWG wire running 3 panels at 30v/3a would create 11w of heat through the main cable, panels get 79w total but your driver is still pumping 90w, this drops efficiency by 12%.
Give each panel its own 10ft 28awg wire and now you get 86.1w total, upgrade the wire to 24 AWG?now your panels get 88.5w total, 20 AWG? 89.4w total.
Basically don't undersize your wire, if you parallel bridge them from light to light you only calculate the main wire assuming the bridge is short runs, the calculator below will help.
This free voltage drop calculator estimates the voltage drop of an electrical circuit based on the wire size, distance, and anticipated load current.
www.calculator.net