... если я правильно разобрался :)

Example - a 10% or 20% wall loss grind that occurs over a 4" or 6" area will usually give a much higher indication than the same wall loss whereas the grind is contoured out over a much larger area and the flux leakage from the grind is less drastic.

C). It is easier to detect a 5% deep grind in small diameter casing than a 10% deep grind in large diameter casing (with the same wall thickness) - because the ratio of wall loss to cross sectional area is higher in the small diameter tube.
http://www.newtechsystems.com/Articles.htm