I was primarily thinking about the latter. Building a large DC from the ground up means they can better optimize cooling systems (there are even a bunch of experiments with ambient air cooling in cooler climates, schemes were the heat is transferred to users nearby instead of spending energy on dissipating it, ...). Since they are big enough to get their hardware custom made, they can optimize there to leave everything off they don't need, do power conversion at rack level. At least for services they control they have potential for clever cluster scheduling to improve things and shutting unneeded machines off.

Lots of potential for small optimizations that pay off on a large scale, but aren't feasible or worth the effort in smaller setups.