To answer this question, let?s consider an example of a cube.
The surface area to volume ratio of a cube is 6/b where b is the length of the cube.
This implies that the surface area to volume ratio decreases with an increase in volume.
Therefore, when the size of maximum aggregate is increased, the surface area to be wetted by water per unit volume is reduced.
Consequently, the water requirement of the concrete mixes is reduced accordingly so that the water/cement ratio can be lowered, resulting in a rise in concrete strength.
However, an increase of aggregate size is also accompanied by the effect of reduced contact areas and discontinuities created by these larger sized particles.
In general, for maximum aggregate sizes below 40mm, the effect of lower water requirement can offset the disadvantages brought about by discontinuities as suggested by Longman Scientific and Technical (1987).