I have always used a separate module regardless of application size, to me this just seems to make more sense. Given a typical application (such as a blog which seems to be the common howto) you would want to manage (as an admin) users, posts, comments, etc...
If the admin area is just controller, then posts would be the action, but that doesn't seem to make much sense. The admin section is the actual area (not what you are trying to control). You are attempting to control a post or set of posts (the controller). What you are doing to them (creating, editing, deleting, updating, moderating, etc...) would be the action.