A given pressure would cause less force with the smaller cases- all directions. So thats not the limit. The beast must be force and thrust. Not pressure.
I would think the limit with smaller cases is relative to bolt thrust. Maybe because there is less case to grab onto the chamber walls. If so, the bolt thrust generated would dip as the case area increased to grab the walls, until the area of the head is large enough to again, begin increasing thrust. Also, eventually too much case gets exposed outside of the confines of the chamber, and SAAMI limits start going back down. Just a theory, but it would tend to explain the non-linearness of published pressure limits.
