Five vowels on a perceptual axis. One force pulls them toward the centre — effort, which likes the tongue at rest, near ə. Another pushes them apart — contrast, which only survives if the vowels stay distinct. Drag the balance and watch the inventory spread out or huddle toward schwa.
The inventory's width doesn't jump from “full” to “reduced”: it eases down as effort gains weight. There's no magic threshold where reduction “switches on” — just a curve, which the width follows by a gentle cube-root law.