Modify Zoom effect (Posts)

This page describes how the zoom effect can be disabled. However, I would not turn off the effect, but modify. Instead of a magnification of the image with greater outer dimensions I want the dimensions remain the same, but the image area is enlarged. It should look something like the hover effects at the pictures on this page.

I tried something like written here but obviously I do not match the right selectors or classes. What do I have to modify in my custom css?

Thanks in advance!