As far as I'm aware, there is no hard & fast grammatical rule dictating whether in or of should follow the word 'reduction' in sentences similar to your examples, but rather this depends on the object being described and the type of quantity which is being reduced.
Usually a reduction in describes a result, where the subject is the quantity being reduced, for example:
This has led to a reduction in CO2 emissions.
Whereas a reduction of tends to place the focus on the reduction itself, i.e. the action which produced the result, for example:
A reduction of violent video games has led to a reduction in violent crime.
But then the above could also be written:
A reduction in the number of violent video games has led to a reduction in violent crime.
A reduction of could also be used in phrases where the reduction itself is the subject of the phrase, and not the quantities being reduced, for example:
Prepare the sauce from a reduction of red wine & vinegar.