More water takes more space, and perhaps the higher temperatures make it easier to reuse the heat? And you probably don't want your water to get above 100 degre

More water takes more space, and perhaps the higher temperatures make it easier to reuse the heat? And you probably don't want your water to get above 100 degrees C, because then you need to deal with pressure. Sand is also a really good insulator if I'm not mistaken. That could also be a factor somehow.