The macroeconomic policies of one economy affect the savings or expenditure of other countries. Trade and finance link all economies in a specific way. Not all economies can have a surplus. The relation between savings and investment or expenditure and domestic production is not unilaterally determined.
What all these economic points imply is that an economy that has a surplus can adopt macroeconomic policies to increase domestic demand, either by income policy or by using its fiscal space to cut taxes or increase expenditure, while still keeping the basic macroeconomic equilibrium of not spending more than what it produces.