Waterfall_13
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No, it's some kind of nonsense; the model didn't change here (it was initially large, and the animations shrunk it to a normal size), and I also played with a mod that changed zombies to mummies; they also jumped, and the model didn't change.
Added 5 minutes ago
Rakshasas also jumped in Warlords.
Well, then it seems I had some kind of bug related to jumping.
As for the increase in size, I think it's simple. The jump effect has its own size, and it differs from the size of the creature. That's why, at the moment the effect is applied, the model increases in size; when the effect ends, the model returns to its original size.