IN RESPONSE to a reporter's question in last Sunday's presidential debate, President Reagan asserted that "the population explosion, if you look at the actual figures, has been vastly exaggerated, over-exaggerated." A look at the actual figures, however, should persuade most people that it is hard to exaggerate the population pressures that now threaten much of the developing world.

Despite progress in restraining birthrates in many countries, the number of people added to the globe increases by a record amount each year. That's because growth rates are still high -- and even rising -- in many countries, and because the rates are multiplied against an ever-expanding population base. As a result, the World Bank recently warned, world population will more than double to at least 10 billion people by the middle of the next century unless far stronger family planning efforts are made.

The president cited as the source of his optimism "some pretty scientific and solid figures about how much space there still is in the world and how many more people we can have." But much of that space isn't inhabitable by any stretch of the imagination. Other areas are needed for farming and the maintenance of natural resources -- soil erosion has already become a critical problem in many developing countries. Nor would affluent nations, such as this one, welcome the enormous influx of immigrants that would be necessary to relieve overcrowding in other countries. If the next 5 billion people were to be evenly distributed across the countries of the world, accommodating them in so short a time would still be difficult. But most of the newcomers will be added to the developing world, where economic pressures already are acute. Some countries, especially in Africa and Latin America, will see their populations multiplied several times during the next 65 years. To avoid social and economic collapse, these countries will have to create jobs, educate young people, and provide health care on an unprecedented scale.

Economic growth will certainly help many developing countries, but the countries with the most economic success are also those that have begun, with the help of active family-planning programs, to curb population growth. In Central America -- which, as the president acknowledged, will put the most immigration pressure on the United States -- the burgeoning youth population has already contributed to disastrous levels of unemployment and continuing social unrest.

White House denial of the existence of population problems has had one healthy effect. It has reminded Congress and the public, which heavily supports U.S. population aid to countries that need and want it, of the importance of such help. As a result, Congress this year voted to increase overseas population aid programs by more than 20 percent. That's an important boost, but still far short of what is needed.