Tricuspid Valve leak
The right atrium is 'the first chamber' of the heart
where blood collects from the entire body.
Blood then travels from the right atrium to the right ventricle through the 'tricuspid' valve. If there is any backflow of blood back through the tricuspid valve then that would cause the right atrium to enlarge. Since the enlargement is only mild then the valve insufficiency (leak) is probably mild.