Dogs have the greatest variances of size within a species. I always think of this picture when I remember that little fact. I never thought I'd see a picture of humans to rival it.
Sultan Kosen (8'1") and He Pingping (2'5"). Tallest and shortest men able to walk, 2009. He died March 2010.
However, dogs still win because the Great Dane and Chihuahua above are healthy breeds whereas both gentlemen have growth hormone abnormalities (gigantism/dwarfism) to explain their respective enormous and diminutive proportions. Still, it's a cool comparison.