You are absolutely correct. However, the bullets Oswald fired were larger than the entry hole in the skull.....
The bullets Oswald fired were nominally 6.5 millimeters in diameter (although actually slightly larger at 6.75 millimeters to allow the rounds to expand against the concentric rifling grooves inside the gun barrel). The entrance wound on the back of Kennedys skull was 6 millimeters wide.
Which somehow disappeared. There is NO way his skull and autopsy information just disappeared. I've never had much interest in this before this came out. Makes perfect sense. Secret Service just can NOT tolerate it coming out that THEIR agent accidentally shot the man he was guarding. No government ever would admit such a thing.