ILM - Index size doubles during shrink process

Hi -

I've noticed the index size when moved from hot to warm phase doubles up before shrinking in size. Just curious to know if this is the expected behavior and if there is a way to avoid the doubling up part of that from happening? We have sized our key to 300GB in size (3 shards, 1 replica). And when ILM kicks in, the same indices are increasing close to 700GB and shrunk to 1 shard, 1 replica. We are going to reduce the primary shard size to 100GB instead with same existing shards soon.

Thanks for your time.

Yes it's expected, Shrink index API | Elasticsearch Reference [7.11] | Elastic describes it a little more.

Thanks much!