There were strong reasons why a State should not be over-populated, and only one reason why it should not be under-populated.

That one reason was the danger of annihilation from invasion.
Sparta was said to have suffered thus, because of under-population, and passed a law encouraging large families.

Alexander encouraged his soldiers to intermarry with the women of conquered races, in order to diminish racial differences and antagonism, and Augustus framed laws for the discouragement of celibacy, but no law has ever been passed decreeing that individuals must mate, or if they do mate that they shall procreate.
Malthus, the great and good philanthropist of Harleybury, a great moralist and Christian clergyman, urged that it was people's duty not to mate and procreate until they had reasonable hope of being able easily to rear, support, and educate the normal family of four, and, if that were impossible, not to mate at all.

As a Christian clergyman, Malthus did not interpret the Divine command apart from the consequences of its literal acceptance.
"Be fruitful," meant to Malthus reproduce your kind,--that implied not only bringing babies into the world, but rearing them up to healthy, robust, and prosperous manhood, with every prospect of continuing the process.
"Multiply and replenish the earth" as a command to Noah, meant in the mind of the Rector of Harleybury, "People the earth with men after your own image." Very little care would be required in Noah's time, with his fine alluvial flats, and sparse population, but in Malthus's time the command could not be fully carried out without labour, self-development, and "moral restraint." The physiological law is simple and blind, taking no cognisance of the consequences, or the quality of the offspring produced.

The divine command is complex.