My question is wouldn't the firing "stretch" the cases to form the chamber dimensions? And if too small(the chambers headspace) wouldn't there be indications of excessive pressure?
Yes, the case should stretch to fit the chamber, no, not if the smith cut the chamber to Min specs. Meaning no room to stretch, a good thing.
Will a small chamber have an affect on pressure, no not if it is in the chamber, yes, if it is in the throat.