Overly pronounced hips may be caused by poor diet or a lack of physical activity. As a result, excess fat will settle on the hips, the buttocks and thighs, especially in women. Also our genes dictate where fat is stored. In general, pregnancy and breastfeeding make women naturally more predisposed to storing fat.
While regular exercise can tone your upper arm muscles, it cannot change skin quantity or laxity. Even if your body weight never fluctuated much, your arms may always have suffered from excessive slackening of the skin. Age also affects skin elasticity, causing once firm skin to loosen and sag.