Stretching a character's silhouette is where the fun begins — I like to think of small bust / big-curve proportions as a choreography between underlying bone structure and bold surface shapes.
First, sketch the gesture and the ribcage/pelvis relationship: the pelvis is the engine for those big curves. Make the hips noticeably wider than the shoulders, and give the pelvis a forward tilt if you want a pronounced rear. Use simple 3D blocks or an egg for the ribcage and a wider, flattened oval for the pelvis so the hips read as solid forms. For the chest, treat the breasts as small, soft spheres sitting on top of the ribcage, not as the focal mass — that keeps them believable even when the hips dominate. Emphasize the waist by drawing a tighter connection between the ribcage and pelvis; a strong waist line makes the hips pop more.
Silhouette and clothing are your best friends. Try high-waisted skirts or jeans, belt lines, and dark side shading to push the waist inward visually. Use clear, readable silhouettes in thumbnail stages — if the curve isn't readable in silhouette, it won't read from a
distance. Shading and highlights should follow simple forms: light the cheeks of the butt and thighs, add soft shadow under the hip bulge and under the small breasts to ground them. I usually do a few quick thumbnails, pick the strongest silhouette, and then flesh out anatomy with a solid understanding of weight and balance — this approach keeps the final drawing lively and convincing. I still get a kick out of how dramatically a few silhouette tweaks can change personality.