My findings would suggest that it’s not because of nsisbi per se.
2017:
Currently, the Whonix-Installer for Windows is based on nsisbi, a fork of the often used “Nullsoft Scriptable Install System”, which was required to circumvent the general size limit of 2Gb usually enforced by “NSIS”. Even with that though, a bit of additional modification was required to include all the files required for Whonix into one simple to use “.exe file”. That’s why for the foreseeable future, a disk-space of 8Gb is required during installation, as the Gateway and Workstation required additional compression, since even “nsisbi” has a single file size limit of 4Gb. [Release] Whonix-Installer and UI
2018:
About a year ago, the first version of the Whonix Installer for Windows was made available to the public. Since then, there has been a continuous effort to find ways to make the process simpler, more reliable and less taxing for computers with little disk space. This effort now culminates in the release of a new Installer, build from the ground up with these things in mind. Based on InnoSetup instead of NSIS, it is far better suited to the task of handling the large files Whonix requires. New Whonix Installer Release
On Github (https://github.com/Whonix/Whonix-Windows-Installer):
There used to a version of this installer based on NSIS which has been discontinued, due to stability and a host of other issues.
But maybe you’ll be able to get an more definitive answer from someone involved in the process.