Why when we do concrete compresion strength we used cubic samples 150mm*150mm*150mm not another dimensions?

For compression test of concrete specimen, mainly2 types of concrete specimen are used, Cubes and Cylinders.                                                  

Cube sizes of150mm are more common in Asia and European countries while Cylinders of150mm in diameter and300mm height are common in Australia and USA.

The most common age of testing is7 &28 days, but tests can be carried-out at1,3 and14 days as well.

It has been observed that the probability of weak spots in a specimen is greater when the volume of concrete is larger for a given compression level. This seems true to some extent because smaller specimen can be better compacted, therefore smaller specimen show smaller amount of bleeding hence the concrete would be of better quality and exhibit better strength.

we can do compressive test for1day3days7days &28days depends but in gulf only7 days and28days test is preferred. Compressive tests are done on samples to get characterstic compressive strength of used concrete in seven days it should reach near about70% of required strength and in28 days test result should be100% of required strength.

Depending on aggregate size, size of mould (Cylindrical/ Cube) are specified... Also the other factor is proper tamping of concrete inside mould... 

I am not quoting it from any specs.. If anyone have diffrent view/better answer.. Kindly share