It’s hard to grasp the enormity of “The One,” but an aerial photograph of the largest modern home in the United States provides perspective. Viewed from a drone, the white marble structure once marketed for $500 million looks every bit the fortress towering over the scattered dwellings of a village. Of course, those diminutive quarters are themselves mansions that climb the hills of Bel-Air, houses that are multiple times smaller than the 105,000-square-foot behemoth hovering above them. That such a giga-mansion could be built says much about the asymmetrical excesses of our age — in which the struggle for housing plays out in the valleys as spec homes that only…