I think you have a really good point about circles being smaller, a side effect of publishing only to certain circles will result in less updates seen, no matter how we try to slice it.
I find G+ posts to be generally of much higher quality, especially shared things.
I believe quality of the posts depends on the quality of the people... I had(since I deactivated both Facebook and G+ accounts of mine) nearly the same sets of friends both on Facebook and Google+ the last time I checked them, and the quality of the posts were nearly the same. Actually, most of them were posting the same things to both platforms.
I find G+ posts to be generally of much higher quality, especially shared things.