How can decreased cardiac output be caused by abnormal fluid volume?

Overloading. Low cardiac function may not be able to keep up with too much volume and the fluid backs up in places like the lung, neck, abdomen, and legs. Too much volume can overwhelm a borderline heart already weak. When the heart cannot keep up the needed cardiac output (liters per min) and making the pump work harder increasing pressure inside the heart, and dilating the chamber getting into a bad cycle.
Physiology. The amount of blood the left ventricle can pump (stroke volume) is directly related to "preload" which is based on the venous return to the heart. If volume is contracted from bleeding, diuresis, vasodilation, or poor intake, preload is reduced, venous return is reduced, stroke volume is reduced &, if increased heart rate can't compensate for the reduction, then cardiac output is also reduced.