Split API for a rolled over index

Hi, I have a badly-sharded index myindex-000003 which rolled over from myindex and has already become readonly. I ran the split API to optimize it and the new index size is 4x larger.

  1. Why it is larger? Is it performing a document “defragging” in the background?
  2. What are the next steps to fully replace myindex-000003? Should I delete it and then create an alias against the new index pointing to myindex-000003? Should I wait for the “defragging” to finish?