In the final scene between mcmurphy and the chief the chief says "I feel as a big mountain" why?

The Chief used to say, “I used to be big, but not no more.” He is a big man but the institution reduced him to little more than a shell. The act of escaping, breathing in the fresh air from the real world makes him deel big again.

The chief associated size with power. Until we hear others talking we don't relise how huge he is, he desribes himself as being tiny. And he always says that Nurse R is big. So when he feels as big as a mountain he feels powerful. He has regained control over his life.