An increase in debt leads to an increase in spending, economic activity and, hopefully, growth as well. But an increase in debt makes the existing accumulation of debt bigger. And the existing accumulation must be serviced.
The cost of maintaining debt is a drag on spending and economic activity and growth. The cost reduces growth arising from an increase in debt.
An economy that relies on debt for growth needs increases in debt that are big enough to offset the whole cost of existing debt -- and bigger even than that, to induce growth.
As time goes by, ever-larger increases in debt are required to induce growth. This is why the productivity of debt declines. It explains the decline you see in this "Debt Saturation" chart from Economic Edge:
|Source: Christopher Rupe and Nathan Martin via Joe Weisenthal|