The average athletic American/Brazilian man is around 5'9"-5'10" and 165-170lbs, so a natural LW, which is why LW has always been the deepest talent pool.
From the there you follow the bell curve down either side to shallower and shallower divisions, however as weight increases the chance of being taken by a Big $port (NFL, NBA, NHL) increases, so the weights above LW are generally shallower MMA talent pools than the corresponding divisions below.
On the other hand, fans are attracted to the spectacular and human beings inherently find bigger dudes smashing each other more spectacular, so the UFC is more likely to hire a mediocre WW than a mediocre FW, hence the higher numbers at WW than FW even though in reality the Big $port effect renders 170 a shallower MMA talent pool than 145.