well that is generally down to environmental conditions and breeding
other than that the top has wings and a head attached so it would carry more feathers than the bottom which only has the vastly featherless legs as extras
splitting the sides another way the front would have more feathers than the rear despite being slightly narrower in most cases, this is for the same reasons as top/bottom
as for splitting the sides left/right both sides tend to be around about the same, with environmental factors being the only decider.
is that a sufficient answer?