Hello RawWrangler:
A few thoughts about your questions.
Generally speaking, the larger MTJ tube is a roughly unconstrained space due to its (presumed) larger space and therefore, as you pump your body (genitals) attempt to expand equally into the space.
The skin on each part of your genitals has varying elasticity - for example if one is circumcised, then that skin (scar tissue) is the thinnest, whereas the skin on one scrotal sack is most elastic and generally the skin is most durable (thicker).
Regarding the "long cylinder", although it is unstated as to the diameter of the tube, it's a generally accepted practice for pumping the shaft to use increasing diameters to expand one's girth, rather than beginning with or using only one larger diameter tube.
As we pump and draw blood and other bodily fluid into the penis, the varying thicknesses and levels of elasticity in the skin play an important role in how we "shape" ourselves during pumping.
There are some great - no nonsense articles on pumping that you might find informative at:
https://www.pumptoys.com/lib/library.htm
Especially the article on Proportional shaping & Controlling the 'DONUT' displayed at:
https://www.pumptoys.com/lib/libd3.htm
Welcome to the pumping world! Look like you body responds VERY well to pumping
Rick