Well Renji will be completely in scale as the whole line is 1/6.
As for Zabimaru, it's size isn't really a constant in the anime anyway.
Also i think on other angles it looks a good size, might seem smaller from the front because it's behind him.
They have All 3 of the other Bleach pieces in one unit here and Renji on his own. It looks really big, Ulquiorra and Byakuya aren't small statues by any means.
I have no problems with this pieces, its ----ing awesome lol. More than i could have asked for!