Extra strong Fox was something I always had in mind for this magic trading nonsense, but obvious flaw was obvious:

Nanase muscle + Nanase muscle still probably wouldn't look anything close to this.

So my solution was to draw attention to Nanase's not-so-beefy appearance, and to point out that her strength had some magic to it.

So... Why EXACTLY is this the result? Unclear! And it doesn't need to be clear! It can safely be assumed that it has SOMETHING to do with with trading magic-enhanced strength, sure, but the specifics?

We don't need that technobabble!

I mean, we'll likely get Tedd saying something along the lines of what I just wrote as a vague explanation, but I think it's okay to be vague so long as the things that happen have consistent internal logic.

Which means it's fine so long as nonsense like this happens again in similar situations with magic-enhanced strength, and other spells interacting with it in similar ways.