Do new neurons divide propotionally?
It seem's you are assuming that your / the adult human brain produces new neurons over time- this is (largely) incorrect: neurons are non-dividing cells and are all formed throughout embryogenesis and very early childhood / infancy (see also this question).

Only very exceptions to this are known as adult neurogenesis (generation of new neurons), the most likely or active region in humans would be the hippocampus, however the extent or importance of the process in humans is still not really known. Additionally the hippocampus is an area of the brain that mostly holds memories and is not really related to reasoning skills or body / motor control.