Testers: how do you report a gauge mismatch without the designer getting defensive?

I'm testing an amigurumi pattern right now, and using the recommended hook and yarn, my piece ends up almost 2 cm bigger than the stated finished size. I've learned that just saying 'gauge is off' can come across badly, so now I include photos with a ruler, my hook and yarn weight, and how many rounds in it started to drift. How do you structure feedback like this, and do you think designers should list stitch tension or rounds per inch for amigurumi too?