You might already know this, but I'll restate it here anyway.. One theory of why the dinosaurs ..or at least many of the "well known" dinosaurs we find fossils of went extinct was because of their poorly evolved sizes. Especially on islands smaller is better. There may not be enough food to support such a large animal, so animals naturally select for smaller and smaller species within their breeding group.
It makes sense to me that an animal who's been around for such a long time would be small.