The sloths we know and love are actually just the last few twigs on a highly pruned evolutionary bush of sloths. Historially there were over 50 different species across two families of sloth, and some of the largest grew up to 5 tonnes!
Research published last year in BMC Evolutionary Biology indicates that sloths have changed dramatically during their evolution, showing extremely fast evolution in body size over the last 30 million years. The authors tested different statistical models of body size evolution against data for 57 species of living and extinct sloth, and found that models considering only extant sloths were poor at explaining evolution across the entire sloth tree. Some sloth lineages showed very rapid evolution; The fastest evolving sloth lineages were the Giant sloth clades Megatheriidae and, which showed increases in body size of 126kg per million years. This makes sloths some of the fastest evolving mammals known. Larger bodies may have been favourable at the time because of their benefits for competing with other species, or in enabling sloths to exploit a wider range of habitats.