Mine don't - unless it's a graphical bug. When I'm setting up the Enchant I get a waayyy higher number when I select Grand/Black over a lower grade gem.
I don't have any lower grade numbers in front of me because I'm not currently enchanting, but I got 46% Fire and Shock resists with Grand Soul gems and much lower when I previewed with lower grade soul gems. If I had gone through with the Enchant would the lower ones have produced a 46% Resist? 


I would agree with you, larger souls have increased the bar further than lesser ones for me during enchanting.
It also makes a hell of a difference on your weapons enchant power and armours enchants depending on what soul you select.