Alzrius said:
It's Huge as soon as it gets that 16th Hit Die, at which point it'd have the same statistics as a "normal" Large Elemental; to me, that's a pretty clear indication that elementals grow in power over time, getting larger and stronger as they do (hence why the last kinds are called "elders").
I'm not sure if this would apply to Elemental Monoliths or Primal Elementals, though.
But you are right on; if you read the whole of the elemental descriptions for water elemental you find this clear pattern:
Small Water Elemental - 2 HD - Advances up to 3 HD (Small)
Medium Water Elemental - 4 HD - Advances to 7 HD (Medium)
Large Water Elemental - 8 HD - Advances to 15 HD (Large)
Huge Water Elemental - 16 HD - Advances to 20 HD (Huge)
Greater Huge Water Elemental - 21 HD - Advances to 23 HD (Huge)
Elder Water Elemental - 24 HD - Advances to 48 HD (Huge)
There is a clear correlation between the size of the elemental and its power level; as an elemental grows in power, it grows in size (at least to huge).
Either gaining 4 HD causes an elemental to suddenly metamorphos into a new creature / race
OR
the simpler more elegant explanation, one creature (Water Elemental) can be found as the least variety of the creature (Small) with 2 or 3 HD to the greatest (Elder) at 24 or more HD.
My vote = one "race"
Of course it doesn't matter. I'm not your DM.
DC