The economists who first stated this relationship assumed that the amount of capital available in a given year to pay wages was an unchanging amount.
So they thought that as the population changed so too would the wages of workers.
If the population increased, but the amount of money available to pay as wages stayed the same, the results might be all workers would make less, or if one worker made more, another would have to make less to make up for it and workers would struggle to earn enough money to provide for basic living requirements.