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If the really big creature was created by a Make My Monster Grow situation, then it can also be No Ontological Inertia.
I'd also call it the inverse of tropes like Hulking Out or Miracle-Gro Monster, and in a sense an inherent part of situations where it's a reversible thing- IE, the Hulk's size and strength gradually fade as his anger does, just like they increase as he builds up rage.
Whats the trope for when a really big creature gets physically smaller as its power is drained, as sort of a visual metaphor? It's kind of like Shameful Shrinking, but not the same.