At every point where we get caught in a chain of thought, or in a memory, or get wrapped up within an emotion, we contract. We contract within the body of our experience, and we gather up within ourselves. Practicing expanding awareness can be an antidote to our self-centeredness. Even when plagued with doubt we can make a deliberate, willful effort to extend out and embrace the whole body of our experience. Here we need strong spirit.
Mindfulness is mind open to the full body of experience each moment, the whole sphere.
When we are self-centered our mind is full of self. A mind filled only with self is small-minded. Big mind embraces the full body of experience. Don't wonder where the sphere ends. Don't wonder how big it is.