وَإِنِ انْتَفَخَ الْحَيَوَانُ فِيهَا أَوْ تَفَسَّخَ: نُزِحَ جَمِيعُ مَا فِيهَا، صَغُرَ الْحَيَوَانُ أَوْ كَبُرَ
Translation: If an animal swells or disintegrates in the well, all its water must be drawn, whether the animal is small or large.
Intifakh means swelling; tafassukh means bodily decomposition. Both indicate that the ordinary size-based bracket does not govern. Even a small mouse requires withdrawal of all water when swollen or decomposed.