OT comprehending large application (was What, in detail, the GDK do)

[Patrick <spell_gooder_now spellingbeewinnars org>]

"a hint to 60MB of sourcecode (18.000+ pages
if printed out) "

I am sorry for the quite off topic post, but I can't help myself. I am 
just a part time programmer, I can understand the GTK API but there is 
no way I could contribute anything to the code base. I have been trying 
to learn more about how professionals study the projects they contribute 
to. I have been buying books on UML and software system designs but 
honestly this is all a mystery to me. It does not seem that many 
projects bother to construct UML diagrams and surely low-level one of 
GTK would fill a football field.

Does any one person actually understand 18K pages of this code or is it 
just a case of sticking to a small section? Could any pros out there 
explain how you tackle this monumental task?

Sorry again for going off topic-Patrick