Ṣahwah refers to a raised area on a horse's back, and ṣahawāt is its plural; here it marks the rider's position. Al-khiff describes the lightweight youth, whereas al-muthaqqal denotes someone burdened by weight or a heavy physique.
Al-ʿanīf is a rough or forceful person. Thus al-ghulām al-khiff and al-ʿanīf al-muthaqqal are contrasting descriptions, not merely two names. Both help demonstrate the horse's extraordinary energy.
The contrast is rhetorically effective: a light rider slips off, and a heavy rider's garments are pulled about. Two different effects point back to one source—the powerful motion of the horse.