I would only suggest making groups smaller if they're also made more specific. That would make viewers more likely to click to the next hub as it should be more closely related to the one they just read. Otherwise, just leave them as they are. A dozen hubs isn't too many.
As for whether groups improve traffic...I don't know. They seem to work on me, but I don't really know whether I'm a "typical" viewer. They certainly can't do any harm.