My 18 month journey in chastity started off self locking in a relative standard Cb6000 and then onto much the same size in open metal cages. However once my wife realised the benefits to our relationship she started wanting me to wear smaller cages for a couple of reasons, 1 she was worried about others noticing a larger cage so a smaller one is more discreet, 2, It keeps me locked more securely and she prefers the appearance of me in a micro cage. How many of my fellow Kinksters have migrated towards wearing a small or micro cage.? I doubt I will ever be allowed to wear anything else but a micro cage.
I started with a CB6000S, but my clitoris escaped very easily. So I started looking for smaller cages. Today I use the so-called "negatives", which in this case are excellent for me. I love to use it, I feel comfortable, and I'm sure he won't escape. Also, I made my own design of the cage (there is a link in my signature) and it pushes around 1.8cm (0.70in) into the clitoris.
For the same reasons you said mostly less likely to spot the cage I also like the idea of it shrinking.