I've often seen this shot in scenarios where the conversation in the scene is relatively surface-level, and there's a lot going unsaid. Their mind (the back of their head) is elsewhere, and that's what we as the viewer should think about.
It's CGI. The nipples skinny Tobey has simply don't match the ones athletic Tobey has. You can work out and bulk up all you like, but that won't change the basic form, size, and colour of your nipples.