The size of animals is directly related to the area in which they live. Large wide open spaces, that required massive movements of the herds to find water and food, contributed to the evolution of these large animals. Their size is directly related to their survival. The bigger they grew, the more ground they could cover, and the fewer rivals, and predators, they had to face.
Because they need to grow larger to keep more weight in the southern hemisphere. Since we have so much humanoid obesity going on in the northern hemisphere, we needed something to counter-balance this or else a polar shift would occur. LOL!
There were large animals on all continents. The Americas had giant sloths, mastodons, and camels. Australia had creatures like the Diprotodon (a hippopatamus sized wombat like creature, the Procoptodon (a 500 pound giant kangaroo), and the Megalania (a 23 foot monitor lizard). Europe had mammoths, giant polar bears, and several varieties of rhinoceros, and Asia has/had elephants and hippos.
Except for Asia and Africa, these creatures have been wiped out. The cause is still being debated but generally it is believed it was due to human hunting, climate changes, and/or the additive pressures from both.