Yes, heart size can be reversed if the condition is a dilated left ventricle. Cardiac hypertrophy is an increase in weight and volume of the cardiac muscle often due to disease.
The heart can become dilated from overwork. Being overweight as well as other reasons can/will cause the heart to work harder and the left ventricle compensates to increase capacitiy. But if and when the LV over stretches at that creates weaker contractions and can/will cause heart failure.
As an anology: stretch a handspring and the recoil is stronger, but overstretch the handspring becomes flaccid. This is the Frank-Starling law of the heart (also known as Starling's law or the Frank-Starling mechanism) states that the more the ventricle is filled with blood during diastole (end-diastolic volume), the greater the volume of ejected blood will be during the resulting systolic contraction (stroke volume). The above is true of healthy myocardium. In the failing heart, the more the myocardium is dilated, the weaker it can pump.
Taking medication can help reduce the heart's workload by lowering the resistance (dilate vessels to reduce blood pressure). Diabetes compounds the problem and needs to be controlled as well.