Because as Catholics we should always strive for beauty - it’s an inherent part of our faith.
Take a meal, as an example. One can either present it nicely with a beautiful table setting, or just plop it on a dish. Same food, but it can serve to nourish the senses as well as the body.
Also, the way people care for/present themselves says a lot about how they are in other areas of their life. If I see a poorly groomed woman with frumpy shapeless clothes and hair that looks as if she’s just woken up, then I would think twice about wanting to trust her with anything of importance - I would be worried that she would take the same “care” of my business as she does of herself.
It has nothing to do with size. I’ve seen very overweight women dressed nicely in clothing that has some shape and flatters them, and I’ve seen thin women in horrible shapeless denim jumpers worn with sneakers and socks.