It's about maximizing density, which corresponds roughly to 1) minimizing energy by making the aggregate have as low of a center-of-gravity as possible, and 2)

It's about maximizing density, which corresponds roughly to 1) minimizing energy by making the aggregate have as low of a center-of-gravity as possible, and 2) minimizing entropy of the arrangements of the components. Often energy and entropy minimization coincide quite closely but don't exactly share the same minimum nor optimization path.