Split NSPs revert back to original size?

[peanutismint]

I'm trying to split a ~6GB NSP file. Whether I split as a copy or in place, this app seems to create one 4.something GB size file, as expected, but then when it finishes creating the second half of the split, the 4.something GB file reverts to ~6GB and leaves me with the original ~6GB file and another one of the same ~6GB size appended with '-split' filename.

Is something going wrong here? I'm on MacOS High Sierra.