'If anything' means 'If there is any difference at all". For example: "People are always saying that my brother is thinner than I am. But that's only an illusion: he gives the impression of being thin because he's so tall. He is, if anything, a little fatter than I am." This means that I don't think that my brother is fatter than I am. In fact, if there IS a difference in body fat between us - which there may not be - I believe that he is actually slightly fatter than me, not thinner than me.

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