In the movie, a relict population of dinosaurs continued breeding for nearly 65,000,000 years after their kindred became extinct. They have been sequestered away, allowing them to evolve to larger sizes as the competition would be more fierce and would need to grow bigger for the need for resources. Also, 65,000,000 years is sufficient to evolve whole new taxonomic families and orders, as mammoths, sloths, cats and humans sprung from humble roots in a fraction of that time. Since the scope of this "what if" scenario can accommodate many speculations, alleged inaccuracies about size, color, posture, diet, etc., become redundant.