Economies of scale. Debt can grow, as long as gdp growth is faster. You reduce the size of the debt as a percentage of gdp when this happens. GDP growth under Reagan went from -2% in 1981 to over 7% in 1985.
Raw GDP under Carter, grew from 2.0 trillion, to 2.7 trillion. Under Reagan, it grew from 2.7 to 6. Reagan did that in the midst of the most severe recession since the great depression.
It'll be interesting to see if Obama, with his "grow government, increase taxes, increase debt" policies will be able to top Reagan's excellent economic record.