A photographer raises the developing time by half and finds the highlights much denser while the shadows have barely changed at all. Why does development affect the two ends of the scale so unequally?

B. Shadow grains hold little exposed silver, finishing early, whilst highlights gain density

Because there is a different amount of material to work with. A lightly exposed shadow crystal is fully reduced within the first minutes and can gain nothing more; a heavily exposed highlight area has far more developable silver and goes on darkening. Development therefore stretches the top of the range and leaves the bottom where exposure put it.