Buried lines also have the opposite problem, that they're too insulated. I believe I once read that underground power lines generally have to be about 50% thic

Buried lines also have the opposite problem, that they're too insulated. I believe I once read that underground power lines generally have to be about 50% thicker to compensate for the higher temperatures (and resulting increased resistance) that they run at because underground wires can't get rid of excess heat as fast as they would in the open air.