Soft deleted documents shows incorrect count

I have a index with 6.9 million documents and did a test to update these documents .

My understanding is that update in elastic search is soft deleting document ,get and create a new document

If my understanding is correct then updating 6.9 million documents will result in 6.9 delete documents but I see only 1 million documents deleted in marvel

Am I missing some thing here or only 1 million documents are updated?

Also index size was 90 GB before update test and now it came down to 9.9 GB and am surprised

can some one give me idea why this is happening

Background merge operations tidy up segments, consolidating old segments into new more compact ones - minus the deleted docs