There’s also the 1936 story “He Who Shrank”, where the protagonist can’t stop shrinking down into successive layers of atom-sized solar systems. He goes through many iterations of this (probably hundreds or more, though the count is never made explicit) before passing through our world (and beyond, because he still can’t stop shrinking). Which incidentally makes his original form, and that of the scientist he was working for, the largest finite organisms ever written about.