Unfortunately, at least according to Mark F, there is no way to alter the block size in HFS+, or even see what it is, although some third party disk utilities might tell you, or some forensic tools. At least I’ve never found a way to do so.
But for Windows, I would certainly go into Disk Manager, see what it is now, and try something bigger, at least 4j (the default may be as little as 1K), and keep trying until things bottom out. Windows tends to use rather small blocks, in order to pack small files more tightly, but with terabytes of space available you are probably more concerned about efficiency.
Of course, if you already have the Drobopro half full, this may be painful, but you might consider buying two or three cheap 500GB disks, or use some you have lying around for such testing.
In any case, please report your findings.