Hip flasks have traditionally been thin, with a curve and size whose shape alludes to the human kidney. The reason for this curve is simple: so that it can easily be pressed against one’s body so that it is better concealed. A curved flask fits better in a front or back pocket as it will press against one’s leg or thigh. Even a flask that is kept in someone’s boot or sock can be pressed against the leg well.
Naturally there are new designs of flask, that include several new designs like the scope flask that incorporates a glass porthole window, letting you keep an eye on your cache.