They’re 20 years old. If you see substantial hips on them now, they’re going to be double-wides when they’re 45. Their hips will widen out plenty when they have that first baby (partly due to fat accumulation, and partly because the ligaments of the pelvis are loosened by hormones that prepare the body for labor and delivery). So count your blessings and enjoy the sight of a girlish figure now, while they’re still girls.
Let me put it to you this way, they don’t look like Raquel Welch when she was in her prime.