If you pick cotton for 80/hr a week I promise you won't be picking cotton in the long run as long as the guy only doing it 20 hr/week (you will develop physical problems very quickly)
In most business you will have a point where the cost per employee align with the output per employee and you try to peg production there. Also getting too many employees can lead to second and third order effects that have to be managed, like transportation of employees and parking.