The ventricles are contracting but the valve won't open unless the pressure in the ventricle exceeds that of aorta/pulmonary artery.
As the blood is not going anywhere the volume remains the same unless there is back-leak e.g. mitral/tricuspid regurgitation where there is no isovolumetric phase.
The isovolumetric phases necessitate that all four (i.e., both semilunar + atrioventricular) valves are closed, so no, volume is not changing.
During isovolumetric contraction, the wall force applied against intraventricular blood is greater, which causes the blood, in turn, to apply greater force against the closed semilunar valves. These valves open when the pressure gradient reverses from aortic > LV, to LV > aortic. This takes a mere fraction of a second and is the first moment of systole, immediately following S1.
Isovolumetric relaxation, which occurs immediately after S2, is the first moment of diastole. All four valves are closed and the myocardium is relaxing. As wall force against the blood is attenuating, force of the blood against the AV valves also decreases, and LV pressure falls below LA pressure, allowing the AV valves to open and passive diastole to ensue.