Why oversize stone or material shall be avoided in sub grade of road?

Because it can cause problems during compaction.

it very hard to get maximum compaction.

Because the road will not be stabile. that because the smal stone will go between the big stone and that will make damage in the road.

because in oversize stone have so much voids space

Because it will case unexpected settlement either through the layers compaction or after aging; which will cause damage of the designed road.