Have a question, some of the pics shown have cages that 4 to 5 inches long when soft and others are like 1 , 2 to maybe 3 soft. I am wondering if the ones that are showing a short dick is because the cage has pushed the 4 inch dick back into the body.
I shall give my two cents on your question.
As others have said some guys are just naturally smaller when soft. Some like the bound or gripping feeling of a smaller cage. Others work towards a goal of shrinking their cocks until they become nubs. While small cock humiliation is also a potential reason.
I personally am of the smaller endowment as well as the tight held together sensation. I tried many different cages before finding the cage that suits me for now. I prefer plastic cages for now. The ability to find the correct sizing is great for beginner training. Silicone was too pliable and I could escape in the night. I have settled on cb6000s. After trying knock offs I invested in a real one and much better quality. And can get different tubes to fit the base.
One of my first purchases was way to large for me soft. My cock also gets rock solid and points straight up when I get hard. So when I started to get an erection instead of being trapped my dick split and broke the tube piece. So that could also be a factor.
Most likely trial and error leads to the ultimate decision.