As the size cube size increases, the strength of concrete decreases due to ...?

By increasing size of cube, surface area contacting with compressive load is increased. Increased surface area covers more voids among coarse agreagate and fine particles.

With application of uniform compressive load, Particles come closer to fill voids and cracks are appeared on concrete at lesser load. This is the reason causing decrease in compressive strenght of cube with increased in size.

IT DECREASES DUE TO 1.) CONCRETE MIX, 2.) LACK OF CURING, 3.) ABSENCE OF REINFORCEMENT.