A specimen of
Armillaria ostoyae
, the so-called dark honey fungus, in Oregon is believed to possibly be the largest living organism on Earth by mass, area, and volume, covering 2,400 acres!
Newborn kangaroos are tiny, only about one inch large! This is because marsupials' young are among the most altricial (underdeveloped when born) of all animals, partly because they are born early in development and partly because marsupials don't have "true" placentas.