Gigantism vs dwarfism: Idea from book 'The Song of the Dodo' by David QuammenReview of Book
Speculation that very early humanoids were giants and later the Maya who survived were very diminutive peoples. Theory
Small kingdoms fought for control of territory and in battle (even in the ritual ball games) the largest males were sent in to fight/defend and defeat the enemy. They killed each other off 1st leaving the lesser stature males to reproduce. In the ritual ball games, it was the winning team that was sacrificed to appease the gods, the strongest and tallest, etc. Also, as food resources became scarce, malnutrition and starvation may have affected reproductive success for smaller individuals who had fewer nutritional requirements to maintain themselves.