The Sony Xperia 1 IV differs from its predecessor in part because the periscope module provides continuous zoom in the 85-125mm range (3.5x to 5.2x magnification).
The Mark 3 module could only switch between 70mm and 105mm; the new one, however, is impressively only slightly bigger.
Additionally, note that the new module has a longer focal length, which may partially explain the size increase.
An Xperia 1 IV teardown is shown in the video below from WekiHome. The majority of the other components—including the headphone jack—are held together with screws besides the back, which should make replacements relatively simple in the event that something goes wrong.