Why did some dinosaurs grow so big?

Paleontologists don't know for certain, but perhaps a big body size protected them from most predators, helped to regulate interior body temperature, or let them reach fresh sources of food (some possibly browsed treetops, as giraffes do today). No modern animals except whales are even close in size to the biggest dinosaurs; therefore, paleontologists think that the dinosaurs world was much dissimilar from the world today and that climate and food supplies must have been favorable for reaching great size.