IF the volume was 1 cubic meter, with sides 1 x 1 x 1 meter
reduce each side by 1/2 means a volume of 1/2 x 1/2 x 1/2 = 1/8 cubic meter
it reduces the volume by 1/8 of the original volume
try it with any other size volume and you get the same 1/8 reduction in volume
original volume = 8, with sides each 2 meters
reduce each side by 50% leaves each side = 1 meter with volume now 1 cubic meter, 1/8 of the original 8 m^3