For why Sue is so big compared to other longnecks, Sue is a Supersaurus, which are big in general (Though apparently not the biggest sauropods). Her line about always being the tallest one is confusing though. Has she not been raised or hung out among Supersaurus? Or is she even big by her kind’s standards? If the latter, that probably would only compound her size insecurities and would be why she would so appreciative of finding a prospective mate bigger than her (Who was based on a dinosaur who turned out not to exist. Whoops! Ah well, I handwave him as a Supersaurus who had a genetic mutation).
As for why Sue didn’t participate in the final sharptooth fight…From a Doylist perspective, the writers wanted the longnecks and the gang to handle them themselves and Sue curb-stomping in would shortchange the other characters of their moment. Watsonian-wise, there can be any number of reasons. She and her mate might have been the last to get up. They could have been farthest from the crater-edge near the sharptooth action. There could have been traffic to that location. Any number of these factors could have made it so by the time she arrived at the edge, the sharptooth fight had already ended.
