Size and Weight:
About the Giant Ground Sloth (Megalonyx):
The prototypical prehistoric sloth, the bones of Megalonyx (better known as the Giant Ground Sloth) were discovered by the American founding father Thomas Jefferson in 1797, in a cave in West Virginia. Honoring its finder, the most famous species is known as Megalonyx jeffersoni, which has since been designated as the state fossil of West Virginia. (The original, Jefferson-donated bones currently reside at the Academy of Natural Sciences in Philadelphia.) It wasn't until Charles Darwin proposed his theory of natural selection in the mid-nineteenth century that naturalists could make sense of giant, extinct beasts like the Giant Ground Sloth (much less the dinosaurs that preceded them by tens of millions of years).
As with other megafauna of the Miocene epoch, it's a mystery (though there are plenty of theories) why the Giant Ground Sloth grew to such large sizes. Besides its bulk, this sloth was distinguished by its significantly longer front than hind legs, a clue that it used its long front claws to rope in copious amounts of vegetation (as big as it was, though, Megalonyx was a mere pup compared to the truly giant Megatherium of contemporary South America). As to whether the Giant Ground Sloth was as slow and "slothful" as modern sloths, that's something we may never know for sure!