Hypokenesis is the medical term for abnormally decreased muscular movement. Wall motion abnormality can/will reduce the EF (normal 55-75).
Idiopathic (cause unknown) dilated cardiomyopathy is an enlargement of usually the left ventricle. I believe the evidence is sufficent to support the dilation is/was due to overstressed/overworked heart.
When there is hypokenesis (poor contractions) it reduces cardiac output. The heart has to work harder and to compenste the left ventricle enlarges (more capacity during filling) and heart rate increases...a faster heart rate may not provide enough time to adequately fill. With medication, the workload for the heart is reduced by dilating vessels, slow the heart rate (provides more filling time)reduce fluids, etc. The heart chamber often returns to normal size.
I was in the same condition 4 years ago as you describe and with medication my EF is now normal at 55% and left ventricle size is normal. I had had a silent heart attack that damaged some heart muscle and reduced my EF to below 29% (heart failure range). Hypokenesis can be in an area or segment and to a degree that doesn't significantly compromise heart contractions and the EF. An EF 40-45% is just slightly less than normal.