Hi Steinar (long time, no see)! You are right that there are very big differences in scope with large features vs small bugfixes.
I was one of the GIMP "Print" plugin maintainers for several years. It ended up being split off as a separate project, turned into a shared library, renamed to Gutenprint, and became the highest quality set of printer drivers on Linux, particularly for photographic printing, due to the use of speciality dithering algorithms (Raph Levien's EvenTone), custom dithering matrices and hand-tuned support for mixing light and dark inks with variable drop sizes, CUPS support, and eventually became the default MacOS X printer drivers in the early years of MacOS X. It went from a tiny plugin as part of a niche application, to being the default means of printing on all Linux and MacOS X systems. I would argue it is many times more successful than the GIMP application, despite being less recognisable to end users, it is nevertheless there under the hood quietly turning rasterised page input into individual ink droplets on pages (or toner on lasers). It far surpassed the quality of most/all commercial RIPs and vendor drivers at the time, and maybe still does.
This occurred because the lead developer of this sub-project was willing to delegate responsibilities and collaborate with others. For example, as a young-but-keen amateur, I single-handedly converted it to build as a shared library, moved the build to use Autoconf/Automake/Libtool (this was 2001, today I'd have used CMake), and this enabled it to be used directly within the GIMP as the Print plugin, or as a CUPS driver, or as an IJS (InkJet Server) driver. That willingness to consider and accept contributions from others made this project an outstanding success. I wasn't the only one either, the project benefitted from the expert contributions from many people with a diverse range of interests and skills, with a good project manager to oversee it all (Robert Krawitz).
In my humble opinion, that is what separates the projects. In order to grow both the developer base and the user base, you need effective project management. And managers need to be able to step back and delegate.
The GIMP developers have never really been able to do this. They have to write every line themselves. This is why its development has basically been stagnant for over 15 years, because they had to do GEGL themselves, even though there's only one part timer on it. Having it all written in C doesn't help. Graphics does lend itself to higher level abstractions which they can't use. I'd consider GEGL a wasted effort myself; it was obsoleted by GPU compute before it was even part way complete.