We're here to make money. Longer Hubs - between 800 and 1,500 words - have been proved to do better on the search engines, so there's good reason to go for the longer Hub.
Another alternative, which works just as well, is to write three or four Hubs on various aspects of the same subject, each around 400 to 500 words long, and interlink them with each other.
In either case, I wouldn't suggest you write "fluff" just to make up the words. The bottom line is that Hubs are intended to be informative, useful articles on interesting subjects. If you can't find 400 words to say about a topic, it's not worthy of a Hub - it's just a blog post.