Legionary short gladius to long gladius

[Bryan]

So in the Republican era, the generally used gladius was between 63-69 centimeters in blade length. Later, it seemed to get shorter, the shortest being the Pompeii style. Late empire it lengthened again, with the usage of the spatha. Is this correct? If so, the Romans spent more time with long blades than short blades. I wonder what the reasoning was to shorten the blade length in the first place? What are the theories on it?