What did malthus think would liimit the population size?

Malthus' fear regarding population growth was that it wasn't sustainable, and that the quantity of food required to feed an ever growing population would soon dwindle.

When plotted on a line graph, this would be the point where the line which represents food overtakes the line which represents the population. When this occurs, the population would reduce along with the food to a point when the population is higher than the food once more.