'he is so full of himself'

What does the phrase "You're so full of it!" mean?
Maybe something like "You're crazy." ?

No, not really 'crazy'. Here the word 'it' substitutes for an impolite word. See
www.phrases.org.uk bulletin_board 22 messages 599.html

It occurs to me that you may also be interested in a similar phrase but with an entirely different meaning; see

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