A better way to think of it- imagine a super 35 sensor camera placed 30cm from someones head. Now to get the entire head in shot at this short distance and with that sensor size you would need a wide 16mm lens. So you're effectively taking a relatively (compared to the persons head) much smaller sensor, and using a lens, stretching the angle/ plane of light hitting the sensor in order to get the head fully in frame.
This stretching due to the distance from the subject / subject size and the much smaller sensor size in relation to the persons head will create a lot of distortion/ warping of the z axis compared to how we naturally see the 3d world.
Now conversely imagine a hypothetical camera which has a sensor size that is 25cm wide- a huge sensor. Still at 30cm from the persons head you'd create much less distortion of the light to get your subject in frame, because the sensor size is now a much more similar size in relation to the persons face.
Or maybe a more simpler way to imagine this- because if the hypothetical sensor was the same size as the persons head 30cm away, an equivalent hypothetical scenario would then be to take a standard super 35 sensor and shrink down the persons head so that it is roughly the same size as the super 35 sensor- so roughly the head size of an action figure, still 30cm away.
In this scenario to match the same framing as before so that you completely fill the frame with the persons head, you'd have to use an approximate focal length of 50mm. And here you'd have no stretching or distortion, yet the distance from the subject hasn't changed, only the relative size between the subject and the sensor. So in this scenario you're distorting light by effectively cropping / zooming in (which won't create distortion), which is what gives you a shallower depth of field. Compare this to the first scenario where you're distorting light via a lens by zooming out (increasing the fov). This is why some directors are saying they prefer full frame because at the same distance from the subject, they don't have to use such a wider lens due to the relative size between subject and sensor size, creating less unnatural z axis distortion.
The amount may not be huge when comparing super 35 to full frame but it's bound to make a difference.
Here's 2 screenshots I took from a video comparing the Alex 65 and mini.
In this test the angle of view is matched by focal length compensation. Ignoring the vignette on the 65 and softening at the edges of the mini, notice how the image on the 65 looks more natural, where as the entire image on the mini is much more distorted- you can see this most on the pillars. It's best to have both images downloaded and flick back and forth using arrow keys to best see this- on the mini the pillars and entire image feels like it's being 'pulled'- which it is because of the wider focal length of the lens producing more distortion.
So you get a much cleaner less distorted image at the same distance from the subject.