+1 to what Eric said!
If it's a good chorus that retains the low end (most of them do... it doesn't have to specifically say "Bass Chorus" to work on bass), it will "fatten" your sound through the doubling effect. Chorus pedals simulate multiple instruments playing in-sync by overlapping the bass signal with a slightly-detuned copy of that signal.
Just think of the violins in an orchestra. They usually play together for an awesome effect. That's a multi-voice chorus right there!
Most chorus pedals are two-voice (original signal, effected signal, mixed together). Some can do more voices (like the Digitech Bass Chorus, with multiple effected voices).
Octavers can also "fatten" your sound if they're the type that overlaps your bass signal with a second signal that's tuned down one octave (or sometimes, two octaves). You might not be looking for this kind of pedal, though.
It really all depends on what you mean by "fatten". Heck, you could also "fatten" your sound with some EQ adjustments that favor low mids and bass frequencies!