I've done a bit of studying on the etiology ("causes") of narcissism and there appears to be a dispute about it from the experts. However, there is one theory (have to look up the authors) who describe self-esteem as internal and external. Keep in mind that the ego is that aspect of the psyche which seeks to protect the individual and keep it viable within the environment in which it finds itself. Thought of in another way, it's a support for survival. If a being lacks a sense of self at all, there is no impetus to maintain life. That is the ego's job. Given this definition, which is contrary to the common wisdom, there is no such thing as a "big ego." That term normally references someone with narcissistic tendencies.

Back to narcissism, if a person fails to develop an internal self-concept/esteem but is raised to be dependent on an external concept, then one is constantly craving attention, admiration, agreement, love in compensation for a poor sense of it (or ego, if you will). This explains why someone can have a very weak ego and yet a (theoretically) strong one at the same time. Personally, I believe this explains narcissism better than just about any theory I've come across.