Question
In a concrete compression test, usually 150mmx150mmx150mm concrete cube examples are used for testing. Why isn't 100mmx100mmx100mm concrete cube samples used in test in its place of 150mmx150mmx150mm concrete cube samples?

Answer
Essentially force supplied by a concrete compression machine is a specific value. For regular concrete strength application, say under 50MPa, the stress produced by a 150mmx150mmx150mm cube is enough for machine to crush the concrete sample. However, but designed concrete strength is 100MPa, under same force (about 2,000kN) supplied by machine, stress in a 150mmx150mmx150mm cube is not enough to crush concrete cube. So, 100mmx100mmx100mm concrete cubes are used instead to increase applied stress to crush concrete cubes.

For regular concrete strength, cube size of 150mmx150mmx150mm is before now enough for crushing strength of a machine.