Why did Malthus assume linear increase in food?

JF comments on Malthus, which is my excuse for raising a question I’ve wondered about for a bit (without being sufficiently interested to actually read M, so for all I know I’m perpetuating common misreadings): M assumes exponential population growth (for which there is some basis) and linear food growth, for which there is no apparent basis at all. To first order (in an economy with little mech), it would seem to be natural to assume that food output is proportional to the number of people working the land. Or, if you assumed that land output and land area was fixed, you might assume that food output was fixed. What assumptions does M use to justify a linear increase in food production?

Refs

* [2018] CIP has a go at the subject.