If you want to praise a girl for being thin, you can say that this girl is as thin as lightning, because lightning is already thin and can be compared with lightning. Then this girl must be very happy.

Moreover, we can compare the thinness of a girl through external objects. For example, this girl can wear the smallest size clothes and is very comfortable, which means that she is really in good shape and really thin before she can wear them.

This is sometimes we can say that this girl is angular, because she is too thin, so all her bones have fallen out and become angular, which is another kind of beauty.