Are the mockups based on my actual trim size and spine width?

Yes. The mockup renders your book at its real dimensions — your trim size, and a spine thickness computed from your page count and paper. A 120-page novella and a 600-page epic come out as visibly different objects, because they are.

This is why the mockups look convincing rather than like a generic stock template. If the thickness looks off, it's almost always the page count — see Why does the book look too thick or too thin?