Improvement in chess is not directly proportional to the number of games played - that is one factor. Amateur park players and hustlers rack up many hundreds of games per week, and thousands per year, but are usually beaten by masters who have played fewer games for shorter periods of time. Botvinnik famously recommended that a professional chess player should only play 80 tournament games a year and leave himself/herself time to think about chess.
Improvement is a function of directed study of chess : analysing one's games with the aim of eliminating weaknesses in one's play, becoming familiar with common patterns (and anti-patterns) of strategy and tactics, learning the fundamentals (and advanced aspects) of openings, middle games and endings, and conquering psychological barriers.
I say this because we need to take this into account as we study perfomance else we get skewed results.