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If thiccness is a function of weight, Primal Groudon or Cosmoem/Celesteelia is the thiccest depending on your definition of legendary.
Width isn't given for most pokemon, and extrapolation from height would lead to autistic game-theory-tier pixel measurements but would be theoretically possible. Even then claiming that a creature which practically or a god retains fat, or and biological features, is a bold, if not unreasonable, claim.