There is nothing saying it is the biggest bug there is but there are several relevant quotes:
Quote:
They are amongst the largest bio-creatures yet encountered amongst the swarms of the hive-fleets, comparable in size and power to an Imperial Titan
Quote:
The Hierophant is the Tyranids' largest shock troop
As such the idea of the "Hydraphant" or whatever as just a bigger version of the Hierophant doesn't work, as anything bigger can't be in the same "Shock Troop" role as the Hierophant.
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As regards to suggesting the scythed and barbed hierodules are the equivilent of warhound titans, there is nothing to suggest that. They are significantly smaller, and are several times described is being roughly equivilent to super-heavy tanks;
Quote:
Broadly equivilent to an Imperial Guard Super-heavy tank, a Hierodule is unstoppable to all but the heaviest weapons.
Quote:
It is thought that the Hierodule is broadly equivilent to the Imperium's own Super-heavy tanks, and fulfills a similar role
Equally, even if it was warhound-sized, that doesn't mean we could simply switch to the Hierophant stats, as there's much more to statting them than just their size.