The rocks at the bottom of a soil profile are big because they are the parent material from which the soil is formed. Here's why:

* Weathering and Erosion: Over long periods, rocks at the Earth's surface are broken down by weathering (processes like freezing/thawing, rain, wind, and chemical reactions). These broken pieces, called regolith, form the basis for soil.

* Smaller Pieces First: The smaller fragments of the rock are more easily moved by wind, water, and gravity. This means the finer particles are carried away, leaving behind the larger, less weathered rocks at the bottom of the soil profile.

* Soil Formation: The smaller particles that are transported away contribute to the formation of the various layers (horizons) of the soil profile above. These layers are richer in organic matter, minerals, and have different textures than the parent rock material below.

In summary: The large rocks at the bottom of the soil profile are remnants of the original rock from which the soil was formed. Smaller particles have been moved upward and contribute to the development of the soil layers above.