Fakhidh means thigh and fakhidhān is its dual. In this line, al-naḥḍ means flesh, not the ordinary sense of rising. Munīf characterizes a lofty structure; mumarrad conveys a smooth, polished surface.
Ukmila is a passive perfect verb, with al-naḥḍu as its deputy subject. The statement is more precise than “large thighs”: the thighs have well-developed flesh and appropriate fullness.