Definition of 'I bet/I'll bet'\n\n You use I bet or I'll bet in reply to a statement to show that you agree with it or that you expected it to be true, usually when you are annoyed or amused by it. [informal, spoken, feelings]

The phrase I/ I'll bet you do,  means I really believe you, so much that I'm willing to make a bet on it.  The scenario in which it's stated, however, is what actually determines the meaning.