From Flora Ames
Mammals were generally pretty small when dinosaurs ruled the Earth; they lived in water or underground and they were adapted to eat a variety of foods. When the asteroid struck, the Earth heated rapidly, plant life died away and large animals were left hot and exposed with nothing to eat. Mammals were sheltered; they could regulate their body temperature; and they could eat insects, helping them to thrive in these new, challenging conditions.
Answered by Laura Mears for Brain Dump in How It Works issue 104