Let's start off with a common example. You like your stream to have some activity, so you've Circled quite a few people. Hundreds, thousands. But you want to use Circles as a means of being well organized. You've got Family, Friends, Business, Acquaintances and Following. Then you add in a couple of interest-based Circles. You add Science! and NSFW and Noisy and Anime and +Felicia Day .
Now, like most, you've also got a smartphone or tablet, and you've set it up to sync with your Gmail account, and you've installed the Google+ app.
Oh hell, now my phone, in Contacts, is listing EVERYONE, wth.
Worry not, there is a solution, but it's not apparent. Let's address the central issue first.
You've seen it before, in the drop down of what you want to share, in the privacy settings on what you want to expose to others. Your Circles.
That's everyone in your Circles, right? No, not necessarily. It's poorly named. It should be Your INNER Circles because in your Google+ settings, scroll down. See that there, Customize Your Circles.
Those with the checks are in Your (Inner) Circles. What are the features of being checked, besides being included when you share to Your Circles? (Worth nothing that also means that when you share to Your Circles you are not sharing it to the ones unchecked here. And that rule ripples outward. Extended Circles is Your (Inner) Circle + Their (Inner) Circles.
But what else?
Well, those in Your (Inner) Circles are synced to your phone.
On ICS+ devices running stock, you can further customize which of those synced Circles is displayed. That functionality doesn't seem to be present in non-stock and older Android devices.
There are also a number of settings which default to Your (Inner) Circles, like who can Chat with you, or send notifications.
It's worth your time to explore. But this concludes the reason for this particular missive. Have too many Contacts on your phone, uncheck a few of those Circles as per the dialog above.