A matter of scale
If terrestrial lightning produces features a metre or two across, what voltage is required to produce the spiders of mars - several hundred metres across, or these larger features you are now speculating about - km in scale.
Reality check - has anything like these been observed? Look at Jupiter's atmosphere where massive electrical discharges occur. How big are these?
(And be aware that we can detect these giant discharges by radio, so giant ones on Earth would be a bit obvious, if they occurred)