In order to keep a plant growing and thriving you must deadhead the old blossoms and deadwood leaves. To keep your story healthy and thriving you must ruthlessly prune unnecessary words and pluck out those characters you don't really need.
My garden always has geraniums in it. I love the plant. The shape, the color, the scent. When I lop off the dead flowers I get to smell the earthy goodness that the plant has buried deep in its core.
When you get rid of the cast of thousands that are inhabiting your story, you will find the earthy goodness. The idea at the core that made you want to write it in the first place. It is no longer being pollinated by every Tom, Dick, and Louis that flew into the garden.
Oh! Wait! You want to give them their own stories. Fine, but they need their own root system. Not every character needs to be seeded in the same plot.