There is a huge difference in power-draw between 2.5" 5400rpm HDDs that one would typically find in a laptop and 3.5" 7200rpm HDDs that are often used in servers even at similar storage capacities. The latter typically use 10W each during normal operation and even a bit more during spin-up. But if you want to optimize your NAS for low power consumption you can use those small 2.5" HDDs just fine (of course being aware that these are not built to withstand the same kind of (ab)use as HDDs specifically built for NAS or datacenter use).

Edit: of course these small drives are not available in more than I think 4TB right now, and if you want to built really large pools of tens to hundreds of TB then the bigger 3.5" HDDs at some point become more power efficient for the same storage space.