Our Ego is the ever-vigilant defense system that works to protect our personal myths.
When people (even ourselves) or circumstances around us call into question the way we think of ourselves it is our ego that sets about assigning meaning to events, justifying our decisions and our beliefs, discrediting any critics, and attacking the source of actual, imagined, or implied criticism or even thoughtful, honest reflection that doesn't match up with our view of ourselves.
Our Ego doesn't just protect our personal myths by destructive means, not at all, it regularly weaves a rich tapestry of illusion to keep us comfortably believing in our personal myths.
Sometimes our Ego is very pro-active. We sometimes feel the need to prove things about ourselves - almost like a defense against our own self-doubts or the perceived/anticipated doubts of others.
People with "big egos" are often compensating for feelings of inadequacy - they build an elaborate defense mechanism with many hair-trigger sensors and proactive reality distortion projectors so that even the casual passer-by gets a dose of Ego sprayed upon them.
See : When fear controls our decisions...
Sometimes people with a strong sense of self, or a solid knowledge of their strength and abilities are labeled as people with "big egos". There is a very key difference between people who just happen to know that they are amazing and potent people and people with a "big ego". It is the difference between a person's self-knowledge and their personal myths.
One of the strangest things about Egos is that they work in both directions - protecting the things we value in ourselves *and* the things we don't like about ourselves. Sometimes our personal myths are woven by our Ego specifically to keep us from realizing just how glorious we are.
See : Our Deepest Fear Is That We Are Powerful Beyond Measure.