Thin belly should start with the structure of the stomach. If you want your stomach to look like nothing, you should lose both fat and muscle. If you want your belly to look curvy, you need to lose fat, but you need muscles. There is no simple exercise method to make your stomach thin, and you need to combine diet. The same is true of yoga. Your yoga instructor has meat on her stomach, maybe she has muscles on her stomach, or maybe she doesn't pay attention to her diet. But when you say fleshy, it may also be a visual effect. People who really practice yoga can have a full breath and a belly as big as a gong, but they can be folded into a piece of skin.