Because I really trained myself in short stories for a long time before I even looked at novels, I tended to be a very lean writer. Nothing wrong with writing lean, I think, but if it's detrimental to your story, then it's the literary equivalent to anorexia.
Generally, what I find is that I haven't thought through every plot line entirely. Plot lines that I thought were finished aren't at all and will play some bigger part later on, if I let them. I had one character, for example, who figures predominately into the first few chapters but wasn't supposed to make another sound until the last page of the story. Much to my surprise, she turned up many more times and she was needed and useful every time.
I think of a story idea, and I start writing it. The trick is to keep thinking of your story idea the same way you did before you started writing it, when you're still trying to flesh it out. Keep fleshing it out, keep bulking it up. Eventually, you'll know when to stop.