Do these two mean the same thing?

So you say!
That's what you think!

Question

Do these two mean the same thing?

So you say!
That's what you think!

Answer

They are similar but the meaning is a little different. 
"So you say!" This means you don't fully believe the other person. Depending on your tone it can be a bit sarcastic or playful/teasing. 
For example = A: "I'm really good at cooking." B: "So you say!"

"That's what you think!" This is a stronger disagreement and is more direct.
For example = A: "You are going to fail this test." B: "That's what you think." (You completely disagree)

Hope this answered your question! :)

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