Consider this. You're wrapping a curvy vase in thick, luxurious wrapping paper. But it looks lumpy, angular and certainly much larger than it really is. That's because the paper is too structured. Now try it again with tissue paper that molds around the curves. Much better!
It's exactly the same with body shape. A structured, stiff jacket will stand off the curves of someone with an hourglass figure resulting in them looking one to two sizes larger. She needs some stretch and softness to her fabrics. However, someone with a straight up and down figure will look fabulous in a structured garment. When you understand your body shape, it makes it a whole lot easier to choose the right clothes that fit and flatter.