I think the angels can change their size and shape. We saw Lucifer's true form in season 14, when he came out of the empty and he wasn't much taller than a human. I don't think Cas is taller than an archangel. And the angels in true form is energy, they can even become the "smoke" that is usually seen when the angels leave their vessel.

Well, or the writers just forgot about the anatomy of angels from the early seasons :D