So I promised
@evster to do some measurements and was rather surprised after a best effort with a mearument tape (no laser).
As you can see from my table, the Model S has less rear seat headroom but a much wider rear seat cushion. Strangely enough, the rear seats seem similar across the I-pace and the E-pace, making me think they might put in a wider rear seat in production models (the one sent around are pre-production I believe).
Inside width, between the most prominent part of the door panels, are pretty similar front/rear.
The trunk is much bigger in the S as we all know, and I did not even bother measuring the Jag's froot.
Rear leg room is interesting - the S has a very C-shaped back of the front seat (the 2017 premium ones) making rear leg room pretty decent from the cushin, but not so much higher up. The back of the Jag's front seat is more straight.
The I-pace has a ground clearence on air susp setting normal, empty, at 17 cm.
Front seats in the Jag are wider than in the S and a much longer seat cushion.
So, summary: a bit smaller cabin, but not the full 6% that the Jag is shorter than the S in overall length (29 cm shorter). The trunk is much smaller though.
It would be great if somenone could measure up their 3 or other competing CUV/SUV, but don't bother with the X - it is bigger all around.