Megan Permanent shrinkage is widely considered a myth. Yes, a penis packed into a tight chastity device will be a bit small after unlocking, but unless the wearer has been undergoing hormone replacement or something, it will be back to its old size after a few unrestrained erections. Any shrinking in this context is more about the fact that a penis kept in chastity will get softer (and thus smaller while in chastity) than a penis that gets stimulation and room for growth all the time.
And generally, a larger cage is considered less comfortable: the earlier you stop the erection, the less it will fight the cage, and an oversized cage will allow the penis to get fairly hard. Typical sizing advice is "snug when the penis is at its softest".
Next, security. I can stimulate myself to orgasm in a cage that is fairly enclosed and too short for a full erection. If the cage allows for full erection, and is as open as the one posted above, it is basically a genital ornament.
Finally, comfort under clothes. A chastity tube is a lot more rigid than a soft penis, and it is supported mostly by a ring around some sensitive parts. Basically, it does not conform to underwear as neatly as a penis does. Wearing an oversized tube under clothes may cause pressure on a piece of skin that is not useded to handling lots of pressure, and I speak from personal experience that it can be quite uncomfortable.