fillSizeRef property
Shapes whose half shorter side is below this get a weaker fill: fillOpacity × (1 − fillSizeDrop × (1 − min(1, halfMin / fillSizeRef))). 0 disables it.
SwiftUI washes small regular glass less than large glass (Task 17b, measured from the coded captures: the contrast kept behind a 20 pt half-size shape is about 0.30 against 0.25 behind a 150 pt one, and dark capsules come out visibly lighter than dark rectangles).
Implementation
final double fillSizeRef;