Bigger creatures require more food and spend their days constantly eating. Large animal species today can usually be found grazing on grasslands. This strategy is the most effective for large herbivores, but when dinosaurs roamed Earth, grass had not yet evolved.
It’s believed that most dinosaurs were plant eaters, and becoming taller was essential to reach higher trees. Any animal with a large neck needs a large tail for balance. Additionally, taller animals require thicker bones for more muscles to attach to, as well as wider legs to carry the weight. Ultimately these factors created much bigger animals in order to meet the needs that many animals today simply don’t have.