I think there might be something wrong with your tube wall thickness measurement.
Let’s take your inside toilet paper roll circumference as a starting point: 125.6 mm. If circumference = pi * d (diameter), divide the circumference of 125.6 mm by pi (3.14159) to get a diameter of 40 (39.979) mm.
Now, let’s add two mm to that diameter (cross section of toilet paper roll; 1 mm additional thickness on each side). We’re up to 42 mm. Our new circumference is 42 * 3.14159, or 131.9 mm.
That is 6.3 mm larger, and that is what I would expect for a 2 mm increase in diameter. If circumference = the diameter times pi (3.14159), an increase of one mm in diameter should give you 3.14159 mm more circumference. Two mm, 6.28318 mm. But six mm is only a quarter inch, at 25.4 mm to the inch.
If your diameter readings were right, and there’s a four mm increase in diameter (more like two mm tube wall thickness), that would give you your half inch increase. Four mm would increase diameter by 12.6 mm, or roughly half an inch.
Or, if you want to do this in inches:
Take a circumference of 5.0 inches. That gives you a diameter of 1.59 inches.
Take a circumference of 5.5 inches (your half inch gain). That gives you a diameter of 1.75 inches.
So, it took a gain of 0.16 inches (roughly an sixth of an inch, or 4 mm) in diameter to get an additional 1/2 inch in circumference. Again, this makes sense. Pi (3.14159) * .16 inches = 0.50 inches.
It’s still a startling comparison, every time I look at girth in circumference and realize that every increment (fraction of an inch, or mm; whatever the unit is) in diameter is multiplied by more than three in circumference gained.
And then you get into area, or cross section, and yes, the penis is not a perfect cylinder, but when area of the cross section is pi * the radius squared, the power of increases in diameter is shown again. Take half of the gain in diameter (radius equals half the diameter), but then square it and multiply by over 3.