I think it was something like since hanging pulled and stretched from the center/top of your penis the bottom ended up getting thicker. Or that the base is increasing to support the increased load on the rest of your dick. Those are the two reasons I've heard.
The tree trunk is always bigger at the roots. If you will look at the ligs that are at the base area of the penis you will see that much of this structure shifts out with lengthening and this extra structure simply takes up more space. When you stretch your penis you are also pulling out some of your fat pad area and converting it to penis. This area has more fat than your original penis base and therefore results in more girth. IMHO.