Aye most times it is a pure net gain; but considering most Hive Fleets have - so far, been shattered or defeated; at some point there's a net loss at those defeats. One could even argue that some worlds might have yielded a near net loss - for example battling demons might cost the Tyranids more biomatter to take a world than they gain from it. Or at least that they gain from it from a quick feeding before moving on.
But yes, by and large a Hive Fleet is a steady avalanche that grows and grows battle to battle.
|