If you cover one of the objectives on each of the binoculars and assess the perceived magnification, I'm quite sure you will find no difference.
The greater parallax of porro binoculars means that the image of an object at short or medium distance will get closer to the medial parts of the FOV of each tube, compared to a roof binocular (or reverse-porro) where the object will be shown more towards the center of the FOV.
When the object appears to the left in the right tube, and to the right in the left tube, you must converge more with your eyes to not experience diplopia (double-vision).
So what goes on here?
When you converge with your eyes, your brain will recalculate the object size. It will appear smaller and closer.
It is easy to show this perceptual phenomenon with a weak prism from the trial lens kit.
With one eye closed, the prism will only move the image sidewards. No change of the apparent object size will happen, since the prism has no dioptric power.
With both eyes open and the prism base facing laterally, the eyeballs are forced to converge and the object will appear smaller. With the prism base facing nasally (and this may only be possible at shorter viewing distance), the eyeballs can converge less than without the prism, and the object will seem bigger.
So, with porro binoculars, you're forced to converge more, but only at finite distance. For astronomic viewing, there's no apparent size difference.
Reverse-porro binoculars do the opposite. Their construction with a tiny parallax means that they have a built-in convergence, which makes their magnification seem considerably more powerful when looking at nearby objects.
//L