When we push the soil particles closer together, we remove the pore space, and the bulk density will increase.
Compaction increases the bulk density by reducing the pore space.
Compaction can also result in the changing of the proportion of pore sizes. This will often result in excess water retention. Compaction changes the amount and size of pores. See the Compaction Diagram Deep plowing can reduce compaction below the plow zone. It is a measure of the shrink-swell potential, or the volume change of the soil with changes in moisture content.