The GLU library itself is pretty big. It has no real dependencies other than GL, which does mean that sections of GLU implement their own memory management, hash tables, and so on. The subdirs in increasing order of size are:
Not the GLU header file, which is in the top level include
dir. These are the platform-specific definitions needed by the
implementation.
project.c is matrix utilities such as
gluPerspective and gluProject.
Tables of GLU (not GL) error enums and corresponding strings.
The GLU functions that create and render quadrics, in
quad.c.
A lot of repetitive code in mipmap.c for rescaling
image data to generate texture mipmaps.
The code that tessellates concave and/or contoured polygons with
holes into low level primitives.
The README is a good description of what it does and
can be used for. It's amazing just how complicated polygons can get.
And as a bonus for code spelunkers, there's a GLU_TESS_MESH callback implemented here. It returns the internal quad-edge mesh structure generated during tessellation. I've no idea what you'd actually do with this, but it sounds cool.
To me, NURBS are some sort of alien creature from an episode of Dr Who. Maybe one day I'll skim through the 39,000 plus lines of code here.