I was wrong. GIMP and OpenOffice are not really written by "hobbyist" programmers.
Instead, there is a core team of just a few programmers who contribute a large amount of the work. Then, a large number of hobbyists around the world contribute to that body of work.
I had the impression that the work was much more spread out.
Here is Maurits' comment:
GIMP and OpenOffice aren't written by hobbyists. There's a small core team of developers that are paid for and do probably 80 - 90 % of the work. The remaining 10 - 20 % is done by a large group of hobbyists like me who try to free up a few hours in the evenings.
Look for example at the ChangeLog file for the GIMP. You will see that most work is done by 3 or 4 developers.