I’m sorry if this is obvious to everyone, but remember that just because length and circumference are both measured in the same units, doesn’t mean that their growth is equivalent. For a long time, I’ve tried to think of growth in terms of volume, like remek has been advocating.
I’ve attached a spreadsheet that demonstrates the difference in volume as derived from circumference growth vs. length growth. It is good for translating that intuitive feeling for the contribution of girth to the volume formula into concrete numbers. You can plug in your own size data to make it more specific and relevant for you.
As an example, you can see that volume from length growth is linear. If we use 5 inches circumference as a constant girth, you can see that for every 1/8th of an inch you add in length, your volume increases by 1/4 cubic inch.
When we look at circumference, it is a different story. If we use 6 inches as constant length, you can see that if you increase your circumference from 4.25” to 4.375”, you gain 0.515 cubic inches in volume. As you can see, if you believe that volume is the only way to equate growth between the dimensions, at that size it would be twice at hard to gain circumference as length.
But it is even more difficult than that, since volume changes as the square of the radius (which is a linearly related to circumference). If you increase your circumference from 6” to 6.125”, your volume increases by 0.724 cubic inches. Of course, by this logic of equating length and circumference, the more girth you have, the harder both length and girth gains will be to achieve, relative to length gains, alone.
Mostly, it is important to understand that the smallest differences in girth, which may be easily overlooked, can make a big difference in volume. Think about that next time you wonder if your erection quality matters during sex. ;) It should be harder to gain, especially over time.
The interesting thing, is that if you made a rough assumption by looking at total average gains of the entries in Size’s DB, the ratio of girth to length gains floats around 0.48, which is conveniently near our example dimensions. If jelqing is considered our most basic and popular exercise and it results in both length and girth gains, it would make sense that the ratio between the two would be nearly equivalent to the relationship between the two in terms of volume. Not a proof, by any stretch, but evidence, if you happen to be looking for some. :)