i have pieces that only clump and curl beautifully at the ends, too. and the rest of the strands look stretched and un-clumped. not exactly what you're describing but weird nonetheless.
i'm assuming it has to do with how i apply product. concentrating on the ends to keep them protected and stop them from wrapping around each other (i.e. tangling). my hair is thicker at the roots but that's because it's new, undamaged growth. and hasn't learned to behave yet!
low porosity, normal elasticity.
co-wash: suave naturals daily clarifying condish with drops of rosemary oil.
detangling conditioner: tresemme naturals.
deep conditioner: curly kinks curlycue renew, b.a.s.k. cacao bark, honey/evoo.