To understand why dinosaurs were so large we need to turn the question of their large size on its head and ask why the animals of today don't grow as large as the dinosaurs.

As any animal becomes larger its mass (and hence its weight) varies in relation to its unit length cubed, while the area (and hence the strength) of its legs varies in relationship to its unit length squared. This mathematical relationship governs the size of all life, so as animals increase in size their strength-to-weight ratio reduces until they reach an upper size limit.

This upper limit is the reason why it is so surprising that dinosaurs were so gigantic. Today's life appears to have reached the maximum size achievable, but life during the dinosaurs' time was much larger. This larger size could be explained if dinosaurs weighed less because gravity was less at that time.