I think no one among the manga authors had a clear idea about how big the 'mountain' of gold should have been. In End it's taller than Battler, in Banquet chap 11 the mountain is tall around as much as TeenagerEva on her knees... but then in another scene it seems taller than Eva herself...
I guess not even Ryukishi bothered about digging into those details. After all the point is that there's a huge amount of gold but how much space it takes is never mentioned.
The 'funny' side is that since that gold shouldn't have taken much space in Ep 7 the Italians could have hidden it in boxes or something like that instead than clearly show them to the Japanese and discharge the closed boxes.
Wikipedia has a page about goldbars, how big they should be and how much they should weight, if someone wants to have fun calculating how much space Kinzo's gold should take.