This is sort of an unofficial poll on two questions. How was it that Dinosaurs could grow so big, and why did they have such small brains. I'll give my ideas of why, please add yours. So big? Because they had gizzards like chickens have. That means they didn't have to use their teeth to chew their food, which mammals spend a lot of time doing. They could just eat and eat and let the gizzard break down the food. Also mammmals have fairly short lives becasue their teeth tend to wear out quickly, but Dinosaur teeth would last much longer. That gave them more time to grow larger. Although some plant eating Dinosaurs had formidable rows of teeth in the skulls. Small brains? Because they were hatched from eggs. Brains use a lot of nutrients and oxygen but eggs only provide a limited supply of both.