For a point source, intensity is given by I=4πr2P where P is power and r is distance.
If the volume of the spherical detector increases by 8 times, then 34πrnew3=8⋅34πrold3, giving rnew=2rold.
The new intensity is Inew=4π(2rold)2P=41⋅4πrold2P=4Iold.
Therefore intensity decreases by 4 times.