Index store size increased by 4X on new index created using slipt api

[sreekanth]

We had one index with 1 shard/1 replica config. As per /_cat/indices, it shows
docs.count =133213
docs.deleted=2430
store.size=819.1gb
pri.store.size=409.5gb

now we used split api to create new index with 40 shards.

    POST /oldIndex/_split/newIndex 
   {
      "settings": {
        "index.number_of_shards": 40
      }
    }

Now newIndex created with 40 shards and came to green status also. And now /_cat/indices shows
docs.count =133213
docs.deleted=404145
store.size=3.9tb
pri.store.size=1.9tb

How is this possible? How can store size grow so big?

[dadoonet (David Pilato)]

Did you also run a force merge after?

[sreekanth]

No, we didn't run force merge. Is it required after using split api?

[dadoonet (David Pilato)]

It's not required but will help IMO. That's at least what I have seen myself recently.