It won’t. After being locked up for a while your first few erections will be smaller than before, but you will return to normal in time.
It does shrink if you are denied erections and your cage is small enough to stop them. But it doesn't cause any physical change. It's just a sponge, that shrinks when it's not expanded, and when you fill it again it'll stretch back to it's normal previous size. After a long period of denial sometimes the first erection isn't as hard or large as previously, but it doesn't take many erections to fully recover. If you really want to keep it small, a short cage and permanent denial of erections is the way to do it.
My longest so far is 70 days without the key going in the lock for any reason, not once. My dick worked just fine and was initially 1cm shorter. Two weeks later and it's 4mm shorter than before the 70 days. If I wasn't caged again I think it would be back to normal without change. My KH said she doesn't care, she loves my dick no matter if he gets a little shorter....which is just as well because age affects size more than chastity and we're all getting older lol.
I was a grower as oposed to a shower.. so i can say my dick has lost elasticity once caged for prelonged period: 6years+ with removal 2-3days interval only during bath. I hvnt ejeculated since my wife n i tried out tis journey. So as a grower, theres ample redundant tissue d dick has when its not in erection.. tat wil b sacrificed by d body once its not in use for long. Erection has goten weaker too, not only in size. Even my wife agreed too. Not tat we r obsesed into measuring its shrinkage but its noticeable.
I am definitely noticeably smaller than I was before we started down this path (although she's never complained about that, since my cock used to hurt her), but the size loss might be unrelated to the cage.