(Small things can get overly large if not watched)My current novel Homecoming is a perfect example of this. As writers, we create whole new universes, and the universe I created in this book is very complex. The backstory is so important to the story, and the races involved have their own way of reacting to things. The possibilities are endless, and I've found myself having to prune my ambitions so that what I want to be an 80,000-85,000 word novel doesn't turn into a 250,000 word leviathan.
Homecoming is about a small part of war on an inter-galactic scale. It's focused on Earth, but it's not confined to it. When the humans try to figure out what they need to do to try and hold onto their reclaimed homeworld, the possibility of engaging with their tormentor on a universal level is countenanced. The main enemy in the book, the Examen, control not just Earth, but several dozen galaxies. Another species, the Traygar, through which we have to pass to reach Earth, is also colossal, and their part in any potential war can't be overlooked.
The problem, of course, is that the sheer scale of these potential events would require more room than any reader would put up with. In order to adequately explain such a war, I'd have to write something that would rival the Bible or The Stand, and I just don't think readers would stick with me that long(or at least not yet).
Therefore, I've had to narrow my focus to what happens in and around Earth. That's hard for me because my personality knows little between zero and 100 mph. Some people do moderation well, but I'm not one of them. Doing so requires exceptional effort on my part since my tendency is to let things grow into an all-or-nothing type of story. I like to think big, but sometimes big can be too much.
So it's like pruning kudzu before it gets out of hand. By staying vigilant, it's possible to deliver a focused story that readers can finish without wondering just where it will end. If I can't do that, then I'm just another invasive species of writer.