In a S.C.'s magazine 1 1/8oz and 1oz loads were tested - the 1oz loads had as good a pattern as 1 1/8 . Because of the higher column of shot , there was more deformation - they colored the shot from front to rear with different colored paint , used high speed photography to do this - the shot at the rear never made it on the pattern circle . That said , a 20ga being smaller in dia. would have a higher shot column than a 12ga , hence more shot deformation - a poorer pattern . My guess is the shot string would also suffer if that last bunch of shot was deformed . Paul