There is plenty of scientific explanation for this and nerve-sparing / non nerve-sapring doesn't make much difference.
When they remove the prostate they also remove a section of the urethra and then join the ends back up. Basically, if you had a quite small prostate about 1" long then your penis will be 1" shorter. If you had a large prostate of 2" long you could lose up to 2". After the catheter has been removed it is important to exercise the penis regularly - pulling, massaging, tweaking, using the pump all help to pull the urethra down and the bladder hopefully comes with it. In some men, the bladder actually settles lower in the pelvis and some research suggests that the more you pump it & pull it, the better your chance of pulling the bladder down permanently.
This is the 'use it or lose it' bit. The cells of the penis are spongy and fill with blood when you get an erection. Without erections, the spongy tissue dies and cannot repair itself. The pump can offset some of this damage if used regularly but the problem is that it is hard to replicate what a healthy penis would be getting - erections during your sleep, on waking, watching the telly, sitting on the bus, etc etc. You can't pump all day & half the night. So the tissue dies.
John no longer has ED but the loss of size is a little bit of a problem - we can't do what were favourite positions because it simply isn't long enough. And he only had a small prostate. Mr P told us that some patients lose so much size that they can't stand up to wee anymore - they have to sit down because there isn't enough length to point anywhere.
It does seem to me that the hospitals & surgeons gloss over this a bit & I don't think the PCUK booklets spell it out either.