That's a feature of phkmalloc (written by Poul-Henning Kamp) that was brought forward into jemalloc (used by FreeBSD and Firefox/Windows, among others) and OpenBSD's malloc (written by Otto Moerbeek et al, borrowing from phkmalloc)
As cperciva already noted below, readlink() is one syscall, whereas reading and parsing a configuration file is many syscalls and much more code. This is malloc we're talking about here, not general purpose user-configurable software.
As cperciva already noted below, readlink() is one syscall, whereas reading and parsing a configuration file is many syscalls and much more code. This is malloc we're talking about here, not general purpose user-configurable software.