"越往下端越细的" can be translated as "tapering towards the lower/bottom end" or "getting thinner/narrower towards the lower/bottom part" in English, depending on the specific context and the object being described.
For example:
This object is tapering towards the lower end. （这个物体越往下端越细。）
The vase has a shape that gets thinner towards the bottom part. （这个花瓶越往下部越细。）