I think most people grasp the basic concepts behind how the foundations of houses work.
The basic idea is you want the mass of the foundation to be set on a footing of sufficient size to support all the weight of the house itself that is to be applied on it----as well as any incidental loads added to the house. 
The actual thicknesses and widths of these foundation structures is a function of soil conditions in relation to projected loads. Most modern footings in this area are a minimum of 10” thick and 18” wide and would only get wider or thicker if there were some unusual soil conditions or concentrated loads (called point loads).
In relation to footings, there is an engineering term called the “angle of repose.” You have perhaps noticed railroad track beds of gravel slope away from the track ties at an angle----not straight down. This is called the angle of repose and is typically no steeper than a 45 degree angle, and if it gets steeper than this vibration and other factors will cause the ties to be undermined.
House foundation footings are subject to the same rules.
As a simple guideline, we always want to see at least a 45 degree angle of support along the footings and away from the foundation. Of course other re-enforced concrete structures can be installed to counter these rules, but that can be the subject of another post.
The home inspector is going to be looking for these kinds of issues when checking out the foundation of the home or its interior support posts which follow the same rules.
In a recent crawl space, a large area had been dug out between two main support posts to create a place to install the furnace. I think you can see from the picture that there is certainly no “angle of repose” to properly support this pier footing and the ground under the footing is subject to collapse. The red line is where the angle of repose should be----not exactly rocket science. The vertical drop from the footing is approximately 4 feet.
This home had two such affected posts and proper supports will need to be installed.
Charles Buell
Seattle Home Inspectors, ASHI Home Inspector, Licensed Home Inspector, Structural Pest Inspector, Charles Buell Inspections Inc, Seattle, WA
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Thank you for the information it is always interesting.