Scale material factor by model size (smaller=denser, larger=fluffier)

Walls are fixed thickness regardless of model size, so small models are
wall-dominated (high material factor) while large models are infill-dominated.

_effective_factor() uses log-linear interpolation:
  factor = MATERIAL_FACTOR - log10(volume / 25 cm³) × 0.15

Example curve:
    0.5 cm³ → 0.76    1 cm³ → 0.71   10 cm³ → 0.56
     26 cm³ → 0.50   100 cm³ → 0.41  500 cm³ → 0.31

MATERIAL_FACTOR env var is now the anchor at 25 cm³ (validated reference).
The card header shows the effective (size-adjusted) factor for the specific job.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

app/estimation.py

@@ -1,4 +1,5 @@
 """Job cost and time estimation from STL geometry."""
+import math
 import os
 
 # g/cm³ per material
@@ -13,29 +14,46 @@ _DENSITY: dict[str, float] = {
 }
 _DEFAULT_DENSITY = 1.24
 
+# Volume (cm³) at which MATERIAL_FACTOR applies exactly.
+# Calibrated to Bambu Lab Studio: 26 cm³ lid → 50 % factor → 16.2 g (actual 16.24 g).
+_REF_VOLUME = 25.0
+
+# Factor change per decade of volume.
+# Each 10× increase in volume → factor drops by this amount (models get fluffier).
+# Each 10× decrease → factor rises by this amount (models get denser).
+# 0.15 gives: 1 cm³ → +0.21, 250 cm³ → −0.15 relative to base.
+_SIZE_SLOPE = 0.15
+
+
+def _effective_factor(volume_ccm: float, base: float) -> float:
+    """Scale material factor by model size using log-linear interpolation."""
+    factor = base - math.log10(volume_ccm / _REF_VOLUME) * _SIZE_SLOPE
+    return round(max(0.25, min(0.85, factor)), 3)
+
 
 def estimate(volume_ccm: float, material: str | None) -> dict:
     """
     Returns a cost/time estimate dict for one job.
 
     MATERIAL_FACTOR (env, default 0.50):
-      Fraction of model volume that becomes actual filament.
-      '20 % infill' in the slicer does NOT mean 20 % of volume is printed —
-      walls, top/bottom solid layers, and shells are 100 % solid and typically
-      dominate. Empirically ~50 % of model volume is printed for typical FDM
-      parts (validated against Bambu Lab Studio output).
+      Base fraction of model volume that becomes filament, calibrated at
+      25 cm³. Scaled up for small models (wall-dominated) and down for large
+      models (infill-dominated). Approximate scale:
+        ~1 cm³  → ×1.4  (tiny clip, almost all walls)
+        ~25 cm³ → ×1.0  (reference — validated against slicer)
+        ~250 cm³ → ×0.7 (large box, infill dominates)
 
     PRINT_SPEED_G_PER_HOUR (env, default 32):
-      Average material throughput in g/h. ~32 g/h matches Bambu Lab printers
-      at normal quality; slower printers (~20 g/h) can be set here.
+      ~32 g/h matches Bambu Lab at normal quality; use ~20 for slower printers.
     """
-    vat_rate       = float(os.environ.get("VAT_RATE", 19))
-    price_per_kg   = float(os.environ.get("FILAMENT_PRICE_PER_KG", 15))
-    material_factor = float(os.environ.get("MATERIAL_FACTOR", 0.50))
-    print_speed    = float(os.environ.get("PRINT_SPEED_G_PER_HOUR", 32))
-    density        = _DENSITY.get(material or "", _DEFAULT_DENSITY)
+    vat_rate      = float(os.environ.get("VAT_RATE", 19))
+    price_per_kg  = float(os.environ.get("FILAMENT_PRICE_PER_KG", 15))
+    base_factor   = float(os.environ.get("MATERIAL_FACTOR", 0.50))
+    print_speed   = float(os.environ.get("PRINT_SPEED_G_PER_HOUR", 32))
+    density       = _DENSITY.get(material or "", _DEFAULT_DENSITY)
 
-    material_ccm  = volume_ccm * material_factor
+    eff_factor    = _effective_factor(volume_ccm, base_factor)
+    material_ccm  = volume_ccm * eff_factor
     weight_g      = material_ccm * density
     print_minutes = round((weight_g / print_speed) * 60)
 
@@ -45,14 +63,14 @@ def estimate(volume_ccm: float, material: str | None) -> dict:
     gross_price    = net_price * (1 + vat_rate / 100)
 
     return {
-        "material_ccm":     round(material_ccm, 2),
-        "weight_g":         round(weight_g, 1),
-        "print_minutes":    print_minutes,
-        "filament_cost":    round(filament_cost, 2),
-        "net_price":        round(net_price, 2),
-        "gross_price":      round(gross_price, 2),
-        "vat_rate":         vat_rate,
-        "price_per_kg":     price_per_kg,
-        "material_factor":  material_factor,
-        "print_speed":      print_speed,
+        "material_ccm":    round(material_ccm, 2),
+        "weight_g":        round(weight_g, 1),
+        "print_minutes":   print_minutes,
+        "filament_cost":   round(filament_cost, 2),
+        "net_price":       round(net_price, 2),
+        "gross_price":     round(gross_price, 2),
+        "vat_rate":        vat_rate,
+        "price_per_kg":    price_per_kg,
+        "material_factor": eff_factor,
+        "print_speed":     print_speed,
     }