My guess is our brains are hyper plastic at that age - new connections keep forming - and total number of neurons does not really increase. So either the the number of connections per neuron increases (this can only happen to certain extent, due to surface area and nearest number constraints - basically you can not arrange a lot of things close enough to have a stonger bond) or you can have connections rewired (break somewhere, and form somewhere else). And my guess is that latter is prefered beyond a certain point - most of activities of babies is sleeping or basic interactions with outside world, and maybe our brain prefers to keep the basic understanding of the world in memory (maybe because that would be revisited the most).

Zipf’s Law is far more universal than human language, It is almost appears naturally if we start with a few simple conditions - consider a finite total group, and a initial distribution, where different elements have different wights and growth rates of elements is proportional to weights. https://en.wikipedia.org/wiki/Zipf’s_law