Along with the loose and comfortable quality, something else is routinely a part of these things. Simplicity. Because the idea of some kind of spiritual practice requires inward turning, the distractions of the outward world are not needed, in fact actively hamper achieving the place desired. So solid colors, clean and simple shapes, and a relative lack of ornamentation are the general rule.
What we get left with is a poetical looking, and nearly sculptural image that diminishes the individual personality, in favor of the journey inward. So we make manifest in physical terms our desire to be aligned with a divine force. What I find most interesting about that, is that we associate closeness to divinity, not with outward display, but with the lack of it. The more centered we become, the less we require ourselves to show to the world.
Take a look at what I mean: