Self is the totality of everything what you know MINUS the dilemmas you have. So it is a dynamic measure: it increases when you learn or understand something and decreases when you face a problem. You feel your "self" increasing when you have solved something and decreasing when you get stuck.
Self-expansion is the process when your self is growing as you have gained further control on your environment. Opposite: self-narrowing = when you feel more and more lost.
Site dedicated to this topic, incl. a book and a video presenting the process of self-expansion resp. -narrowing. Find these in the related links.