There are many reasons more important than population. In Brazil and Argentina's case, they had a mini-colonization by the British in the turn of the 19th century, they created the first clubs, leagues. Football was far more popular in those countries than the northern hemisphere countries like Colombia, because of the British influence.
In simple terms, British popularized football in Brazil and Argentina, and then Argentina popularized football in Mexico, Peru, Colombia decades later. They have a founding father effect in a lot of those countries (not 100% sure about Mexico). In Brazil's case, another curious aspect, it was probably the only established footballing nation that had a sizeable amounts of black athletes competing in world stage. Up untill late 90s and 2000s where we started seeing European teams fielding 3, 4, 5 black players. Of course there were the African nations but they never were organized enough domestically.