What is the difference between ductile iron pipes and steel pipes in resisting external loads?

Ductile iron pipes normally possess thicker pipe walls and are generally stiffer than steel pipes. As such, it replies less on side fill soils to support external loads. Hence, it is not necessary to achieve highly-compacted soils for ductile iron pipes for sustaining external loads.

For steel pipes, owing to less stiffness associated with thinner pipe walls, it relies heavily on the stiffness of backfill soils in resisting external loads. Hence, to enhance the external load-carrying capacity of steel pipes, the most convenient methods are to improve the quality of backfill materials
and to increase the level of soil compaction.

This question is taken from book named – A Self Learning Manual – Mastering Different Fields of Civil Engineering Works (VC-Q-A-Method) by Vincent T. H. CHU.