There is actually no uncontroversial explanation for the as-of-yet unparalleled size of dinosaurs. If you look at the distribution of large sizes in dinosaurs, one distinction becomes clear: the only dinos that were small (less than 1m) were carnivorous theropods. Except for the 70 cm Fruitadens Haagarorum, no herbivorous ornithischians were small.
As Horner explained in his 2009 book How to Build a Dinosaur: Extinction Doesn't Have to be Forever (and in his TED talk on de-extinction), you can turn on dormant genes to get them to build body parts, including a dino-like tail, snout and even teeth. Because of their availability, chickens work well, so Jack intends to build what he calls a "Chickenosaurus."