For freight there are two considerations, what is its weight and what is its volume.
The two major worldwide freight/package carriers give great attention to this in their aircraft selection. Aircraft can "bulk out", that is the cargo area is physically full befor ethe aircraft gets to maximum takeoff weight. Or it can "gross out", which is the opposite, the MTOW is reached while there is still space in the holds. The ratio of cubic capacity per tonne varies of course between cargoes.
A glance at your supermarket fresh fruit/vegetable section will show some long hauls, green beans from East Africa for example. These would not be economic to airfreight by themselves, but because the predominant freight flow is of high value and manufactured goods from Europe down to Africa, there is scope on the return to carry such loads at a significantly lesser rate, and still maximise the overall revenue.