Sagittarius takes the thighs and hips, and in many versions the liver as well, following the sign's rulership by Jupiter, to which the planetary scheme also assigns that organ. The thighs are the part of the body most associated with movement over distance, which suited a sign of journeys and of the centaur's horse-body. Jupiter's temperate and moist nature and its association with the sanguine humour gave the region a generally favourable reading in the older texts, with complaints attributed to excess rather than to deficiency, since excess is Jupiter's characteristic failure. As elsewhere, the specific readings are derived from the sign's ruler and qualities and not from any observation of the part.
Sagittarius stands opposite Gemini on the zodiac wheel, and the two signs between them govern the body's paired limbs, arms above and thighs below, a structural echo later commentators noted without treating it as load-bearing for the doctrine, since the scheme's ordering is sequential rather than built from opposite-sign pairing. The reading of the thighs for complaints of excess follows directly from Jupiter's classical reputation as the most benefic and the most prone to excess of the seven traditional planets, a temperament read into every region Jupiter rules and not something derived independently from any property of the thighs themselves.
Sources AstronomicaMarcus Manilius with Ancient MedicineVivian Nutton