When Hasbro was talking about the new R2-D2 they said that they were very aware of the issues regarding the scale of the first R2 that had been the basis for the Black Series astromechs. So for the new version of R2 they went to great lengths to get accurate measurements of the on screen renditions of R2 so that the new Black Series sculpt would be appropriately sized in TBS. This R5-D4 is clearly the product of that investment into the new astromech body.
BTW, I'm in the same boat regarding the first R5-D4 release - I've got two, and I never opened them either!