All of mine do that. My hair starts to dread from the middle to the tips before it dreads toward the roots. So all of my length gets knotted toward the bottom of my dreads. In time it will tighten and lengthen and blend more with the rest of it. But my oldest dreads are 5 years and they are still thicker at the bottom. That never completely goes away because there is more length that shrank into those few inches and caused it to fatten up at the ends. Does that make any kind of sense? LOL it is really hard for me to explain
when hair starts knotting and the length shrinks up at the ends it is fatter on ends. The new growth of hair knots but there is not the same amount of length to knot up....so it is thinner than the bottom.