Then we have people who're pretty new and moderately active: jhk, gsrb3d, dingto, damien78, Dalai Quintanilha Felinto, nazgul, metaandrocto, ZanQdo, mindrones, nexyon (we're dropping off a big cliff here at this stage - nexyon has ~100 commits over 2 years)
And older contributors who have been quieter recently, but are still around: ben2610, schlaile, genscher, elubie
So that's 7 very active developers, 10 active newcomers, and 4 old-timers who are still coding occasionally. 21 or so.
Yes, they're doing better - they have 10 new developers in the past 2-3 years, to 2 for the GIMP, and they have managed to keep old heads and knowledge around better than the GIMP, but people overestimate the number of active hackers there are working on projects like these.
Blender is seen as an amazing success primarily because Ton does such a huge job of fundraising, planning & doing these amazing projects around film & game development with Blender, and is very conscious of the importance of the artists using the software when planning features. Basically, he runs the Blender Institute as a company, and manages to pay 3 or 4 hackers full time 6 months of the year to work on features - which is an amazing success story - and not something the GIMP has ever tried to replicate.
That said, all the comments saying "GIMP UI sucks, that's what is holding it back" can't have used Blender much - now there's a UI where you really need to know what you're doing to be productive...