Going on 5 years I was in ICU for 3 days due to congested heart failure as a result of occluded coronary arteries. The LAD was completely blocked, RCA 98%, and ICX 72% and the EF was 13 to 29% (below 29% is medically considered heart failure mode). A stent was implanted to the RCA and LAD had developed a natural bypass with collateral vessels, the ICX not stented. I was dx'd with having had a silent heart attack (no symptoms until congested heart failure...shortness of breath and pulmonary edema). Today, my EF is 57%, enlarged left ventricle is normal size and I have no symptoms with medication, and exercise 3 times a week.
If you had a heart attack and quickly treated, it is possible your heart cells were "stunned" and not necrotic. One's EF is decreased when there is heart muscle damage** (stunned, hybernating or necrotic) and the heart's pumping contractions are impaired. If your left ventricle enlarged due to an increase in the heart's workload (stress and strain pumping against constricted and/or occluded vessels, medication, exercise, diet, etc. can/will reduce the heart's workload and reverse the enlargement. An enlarged left ventricle also weakens contractions!
**Medically stunned are heart cells that are not receiving sufficient blood/oxygen and there is short period of time to revive, and that differs from hybernating heart cells that are slowly deprived of blood/oxygen and sometimes can be revitalized with opening a good supply of blood/oxygen to the deficit area. Cell necrosis are dead heart cells and permanently impairs contractions, and may limit recovery, but exercise, medication, etc. can limit any progression to heart failure.