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It's the BIGGEST abdominal showdown ever -- Shaq hitting back at Big Show with some stomach muscles of his own ... and TMZ Sports has the video!

After Big Show posted pics of his own gut muscles last month, some people thought Shaq was backing out of their WrestleMania 33 match because he was scared of the 7-footer's impressive physique.

But Shaq tells us, "Big Show is scared. He called me 'fat.' I am PHAT ... Pretty Handsome And Tall."