It's not the size of the dog in the fight, it's the size of the fight in the dog.

We searched Mark Twain's published books, essays, notebooks, documented correspondence, and speech transcripts, along with the major scholarly compilations of his authenticated sayings, and found nothing containing this line—an unsurprising result given Twain's death in 1910 and this saying's much later documented trail. The strongest early evidence we found is an April 1911 item in "The Bookman," reprinting newspaper material that credited the aphorism to former heavyweight boxing champion Tommy Burns: "It isn't the size of the dog in the fight, it's the size of the fight in the dog." That instance postdates Twain, arises in exactly the fighting/sporting context where the line makes sense, and names a specific competing figure rather than merely gesturing at "phrases like this." Later usages we traced—including a 1958 Dwight Eisenhower address and its long association with coaches such as Bear Bryant—are all firmly in the 20th century and reinforce that the saying circulated as a sports aphorism decades after Twain. Reference works we consulted, including "The Facts on File Dictionary of Proverbs" (Manser and Fergusson, 2007), treat it as a general proverb rather than a Twain original, and no Twain primary source surfaced anywhere. Because there is both a complete absence of any Twain source and a citable earlier competing attribution (Burns, 1911) predating the Twain credit, the evidence points away from Twain rather than merely failing to confirm him. This quote is misattributed.