Mammals grew humongous sizes after the dinosaurs died out.

Researchers have shown that the extinc­tion of dinosaurs 65 mil­lion years ago paved the way for mam­mals to get big­ger  (about a thou­sand times big­ger). Mam­mals grew from a max­i­mum of about 10 kilo­grams when they were shar­ing the earth with dinosaurs to a max­i­mum of 17 tons afterward.

The max­i­mum size of mam­mals began to increase approximately  about 65 mil­lion years ago and slowly died down about 25 million years ago. The largest mam­mal that ever walked the face of earth -- Indri­cotherium , a horn­less rhinoceros-like her­bi­vore that weighed approx­i­mately 17 tons and stood about 18 feet high at the shoul­der -- lived in Eura­sia almost 34 mil­lion years ago. 

In conclusion you can obviously see that after mammals didn’t have to compete with the dinosaurs for survival they thrived and grew to humungous sizes all over the world.