Why do you want to reduce tyre size, lower your ride height, increase wheel arch gap, compromise handling and grip and most of all increase your fuel consumption meaning you will have more revs/rotations in the same distance and your engine isn’t intelligent enough to see the distance because all it does is produce wheel rotations
Its basic maths and physics. Less circumference will mean your tyre will rotate more times to cover the same distance. Manufacturer has installed an optimum tyre size which is supposed to work best with the setup it has designed the car on