The first part is technically true - the automation team can exceed the original team, but this may be a special case of "engineers untangling a mess of functionality accumulated over the years." The second part has an exception of its own - there doesn't necessarily have to be a complexity threshold, developers "over-automate" simple processes all the time (e.g. Microsoft).
Not by definition. Rather, only once it becomes easier to automate than to do by hand, we automate.