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Heldring
As Phil says traditionally they got from steeper to slacker across the sizes (largely KOPS related) the thinking being that as legs get longer with bigger sizes the ST relaxes to compensate and keep the knee in the optimum position.. This also has an effect on seated reach (approx 10mm per +/- degree). Same ST angle across all sizes would be fine too - you'd expect very linear reach measurements to accompany though. The Crust employs neither as dictated by the toptube lengths.
There's some modern thought that KOPS is BS (as exemplified by Keith Bontrager amongst others) and that there should be steeper ST angles used for road in general (esp. with larger frame sizes)... http://cyclefit.co.uk/journal/are-current-road-bike-frame-geometries-outdated