What is the pathophysiology behind global cardiac or chamber enlargement?

The heart is a pump with two circulatory systems in series, a high pressure side, the left, and a low pressure side, the right.
An abnormal connection (a shunt) causes increased flow (volume overload), leading to cardiac enlargement.
An insufficient valve also causes volume overload, leading to chamber enlargement.
A stenotic valve causes pressure overload, leading to hypertrophy (which can be difficult to assess on CXR).