A T.Rex skull wasn't that big. The average Rex skull is only about 1 metre length, with the largest being 1.5 metres. That's shorter than the average human, ergo one human won't fit entirely into its mouth like the lawyer did.
And no, the size wasn't always the same. Go rewatch the movie. The Rex head that exploded through the Jeep roof was noticeably larger than the Rex head that overturned said car 5 seconds later or killed the Velociraptor at the end.