ChatGPT:

I am guessing that the answer to the riddle is the shadow.

A shadow is tall when the object casting it is short and vice versa. A shadow can be big when the object casting it is big, and small when the object casting it is small. Shadows are here, meaning they exist in the physical world, but they are also there, meaning they are on the surface of the object. Shadows are not visible on their own, they only appear when there is a source of light and an object to cast it.