Developing and releasing for macOS was not particularly difficult once a working GNU/Linux version was finished. Both Simply Fortran for GNU/Linux and macOS use the GTK+ toolkit for the GUI version, and their other actual code differences are minimal. The macOS version did require some additional development with respect to packaging the application as well. The largest task was implementing the GUI in GTK+ initially, which was a far more complex task.
The installer sizes decreasing depends on what platform we're discussing. On GNU/Linux, none of the installers actually include the Fortran compiler itself. Those packages rely on the system's Fortran compiler. If using a distribution-specific package such as a .deb or .rpm, the Fortran compiler is automatically installed when Simply Fortran is installed.
On Windows, the installer has decreased substantially in size because of some packaging tricks. Basically all of the space savings came from decreasing the size of the C++ standard library we include. By changing some compiler flags while compiling the... compiler... we've been able to trim the C++ libraries to about 10% of their original size without loss of functionality. Basically, we've stripped out any debugging information and requested size optimization during the build. It isn't ideal if you're using Simply Fortran for C++ development, but I think it's probably beneficial for the bulk of our users.