Dinobots have always been historical depict as bigger than the average Transformers (both Autobots and Decepticons) although the animation errors in the G1 cartoon often cause inconsistence with the size of the Dinobots: in some scenes the Dinobots looked bigger, in some scenes smaller. This kind of errors were frequently in the G1 cartoon, keep in mind that american cartoons in the 80s were made by underpayed Koreans treated as slaves.
Most of the continuities and sagas that came after the cartoon have followed the idea of depicting the Dinobots as bigger than the other Transformers (look at Transformers: War for Cybertron/Fall of Cybertron). The movie just reprised the idea and made the Dinobots even bigger in order to allow the Autobots to ride them and to express their might better.
The Dinobots in the movie have dinosaur forms because they were among the Legendary Knights, the first Transformers made and, as we see at the beginning of the movie, the Quintessons/Creators terraformed the Earth an eon ago and mutated the dinosaurs into the Dinobots. The movie version of the Dinobots are the result of the cyberforming of the Earth's dinosaurs.
