That's a totally different story. There are so many data structures involved depending on what you are doing ie
compression algorithm, number of segments, number of shards...
Comparing MySQL storage (not speaking about MySQL indices) vs Elasticsearch storage is like to me comparing oranges and apples.
If we just speak of storing
_source, which mean basically disabling all indexation on all fields, then you might be able to compare...
Compression on stored fields is done per Lucene segment. A shard may have multiple segments unless you call the
_forcemerge API. An index may have multiple shards.
So if you want to "compare", you may want to create one index with one single shard, disable all indexing for all fields in the mapping, and once the operation is done, call the
_forcemerge API to have only one segments.
Then you can more or less check the size on disk by doing:
du -s data/nodes/0/indices/vj8heygETPqlLrBQUZsR1w/0/index
vj8heygETPqlLrBQUZsR1w depends on your index id, which you can find with the