The trend for smaller PCs for the average user is definitely there, as ITX becomes more popular and more of an option without the associated cost, it's becoming more feasible. Little VESA mountable PCs which still use desktop hardware are pretty cool, and a better option than all all-in-one PC or a laptop for someone wanting a desktop PC, in terms of upgradeability, ease of repair, etc.
As for enthusiasts getting bigger PCs, I don't see that to be honest. The larger cases on the market today aren't significantly larger than those available a few years ago, and they can accommodate oversized boards, 4 graphics cards, and lots of watercooling gear with ease. With SSDs becoming more accessible, there's no need for racks of hard drives in a normal PC, those who need the storage often use a NAS or home server rather than using their main PC for absolutely everything.