what made dinosaurs to grow as big as buildings?

Dinosaurs as big as buses or five-story buildings would not be possible if their bones were dense and heavy like ours.
Like present-day birds, dinosaurs had hollow bones with inner structures known as air sacs, which made their skeletons lighter and less dense. These structures were apparently so advantageous that they emerged at least three times during the evolution of dinosaurs and pterosaurs (flying reptiles), according to a study supported by FAPESP and described in an article in Scientific Reports. “Less dense bones containing more air gave the dinosaurs and pterosaurs [and still give birds] more oxygen circulating in their blood, as well as more agility to hunt, flee and fight, or even to fly. They not only used less energy but also kept their bodies cool more efficiently,” said Tito Aureliano, first author of the article. The study was part of his PhD research at the State University of Campinas’s Institute of Geosciences (IG-UNICAMP).
Aureliano analyzed fossilized bones from three Brazilian species of the Late Triassic (about 233 million years ago), the period in which the dinosaurs emerged. All the bones were found in recent decades in Rio Grande do Sul, Brazil’s southernmost state. Detailed knowledge of specimens belonging to different groups and dating from an early stage in their evolution provides a basis for understanding when certain traits were developed.