Well let's think about this a little. Each stop halves the amount of light, right? Mostly wide open, going from F2 to F2.8 the iris has to close enough to cut the light in half. That's about 1/4 of the total glass diameter. The pins on each leaf need to move around the barrel quite a bit.
At F2.8 the amount of glass being "used" is smaller, and so the amount to cut the hole size in half (still about 1/4 of the total area) for F4 is less. The rotation and pin/leaf movement is less. By the time you get to F16, the hole is tiny. The amount of rotation to move the pins the tiny amount to close the hole that tiny bit.....is small. The aperture lines on the outside are therefore closer together.
I just looked at my F2 Summicron (10 leaves, equal scale distance), and F2 Jupiter 8 (9 leaves, decreasing scale distance). So it's not the number of leaves, it must be something else, like leaf shape. Looking closer, the leaves of the Summicron have a complex curve on their edges. Each edge is curved in one radius, then it has a "bump" and changes to another radius. That must be it. The J-8 leaves have a continuous curve.
