Probably not terribly bloated, just old. The language (C), features (probably K&R C), libraries (pitiful compared to today), and portability concerns (nethack was ported to many platforms, DF comparatively few) it originally had to contend with made for a very different world back then. While I'm sure it's been updated/cleaned up over the decades, it probably still carries a lot of that design/baggage from the time it was written in.