To change the fill percentage you have to change something, if the useable volume increases for a given amount of powder pressure giving a smaller fill percentage pressure is less.Hypothetical scenario: If we change the case fill% by just tinkering with case size WITHOUT changing anything else like seating depth, powder qty etc etc...what should happen to the pressure in chamber? WIth lower fill % will pressure go up or down? what will happen with higher fill %?
If useable volume is the same and you use more of same powder to increase fill percentage pressure goes up.