Ha, i was just going through some science site and they had explained why did dinosaurs grow so biggggggg ?? well , i never questioned this … why they grew so big then i saw Jurassic park movie 
well , according to that site explains reason ,
Sauropod dinosaurs such as apatosaurus are the largest animals ever to walk on land. The heaviest weighed up to 100 tones and the longest might have measured as much as 60 meters from head to tail.
Why they became so big is a long-standing conundrum. In 2002, Jeremy Midgley, an ecologist at the University of Cape Town, South Africa, suggested that it was down to deficiencies in their diet.
In the Jurassic, herbivorous dinosaurs had a serious problem getting enough nitrogen, Midgley argues. "The average nitrogen content of most plants living then was typically lower," he says. "And then the carbon dioxide levels were much higher, maybe 10 times today's. That suppresses the nitrogen content of plants even more."
How does getting big help? As an animal's body size rises, its metabolic rate falls, along with the growth rate. Why this is so is controversial but it does affect nitrogen requirements: the lower an animal's metabolism and the slower its growth rate, the less protein and DNA it has to make, and thus the less nitrogen it will need per mouthful.
Juvenile sauropods must have supplemented their diet with a little meat, and perhaps fungi. Once they had become truly gigantic, however, they could survive solely on low-nitrogen leaves, Midgley believes.
Solving one dietary problem might have created another, though. Jim Elser, an expert in ecological stoichiometry at Arizona State University, Tempe, suspects that giant sauropods had problems getting enough phosphorus.
To support their weight, giants need relatively larger bones than smaller animals. This means they need relatively more phosphorus, because bones are made of hydroxyapatite, a phosphorus mineral. Large and fast-growing plants have a higher phosphorus content, so the giant sauropods may have been able to get enough of this element by eating the leaves of large trees.
Njoy ??? …
