Compressive strength of 100mm cube is greater than the compressive strength of 150mm.
The reason is that..
1) If we mould concrete into cube having a small demension like 100 X 100 X 100 mm^3 the bonding is very good than the 150 mm^3 cube. we know that as the size of the cube or beam increasing the void spaces in it also increases. If there are voids, the concrete element is unabel to take the load.so, small dimension cube will have more compressive strength than the larger one. and
2) we have a relation stress = force/area.
let us concider two cubes of volume 150mm^3 and 100 mm^3 .apply same amount of force ,say 1KN on each cube.then what will be the stresses induced in each cube...
in 150mm^3 44.444 kn/m^2
in 100 mm^3 100 kn/m^2
100 mm^3 is having high stress means high internal resistance,, means resistance to failure. so it is having more compressive strengh than bigger one.