Java garbage collector complains that it is running out of memory on a larger document

When I tried to convert the Valgrind documentation from .fo to .pdf format via fop, this resulted in many garbage collector warnings:

GC Warning: Out of Memory! Returning NIL!
GC Warning: Out of Memory! Returning NIL!

The details are as follows:
$ uname -a
Linux antec-opensuse11beta 2.6.25-rc9-17-default #1 SMP 2008-04-15 22:54:53 +0200 x86_64 x86_64 x86_64 GNU/Linux
$ cat /etc/issue.net
Welcome to openSUSE 11.0 (X86-64) Beta1 - Kernel %r (%t).
$ grep MemTotal /proc/meminfo
MemTotal: 508928 kB
$ rpm -qf /usr/bin/fop
fop-0.94-31
$ svn co svn://svn.valgrind.org/valgrind/trunk valgrind && cd valgrind \
&& ./autogen.sh && ./configure && make -s dist
...
$ cd doc/print
$ ls -l index.fo
rw-rr- 1 bart users 1439984 2008-04-21 18:16 index.fo
$ fop index.fo index.pdf
...
GC Warning: Out of Memory! Returning NIL!
GC Warning: Out of Memory! Returning NIL!
...