Hi chris6507,
I would agree that in larger games the effect of individual expansions/ mechanisms gets reduced. Now, we typically identify our gaming mood in advance and choose accordingly: base and usually a river with only a couple of extra expansions so our interaction focuses on how they play.
We want to fight and do mean things to each other? P&D or Tower of course! Maybe even Fliers too for extra carnage.
Feeling more civilised and constructive? I&C, T&B, Goldmines, Fruit Trees etc.
Feel like some serious road interaction? I&C, Ferries, King & Robber Baron...
I haven't played MegaCarc and it has its attractions, but personally going bigger to feel the impact is just making the problem bigger rather than better.
I agree about Hills and Sheep, it is one of our favourites and Shepherds make an excellent alternative to Farmers if you don't feel like the tension of a farming game...