How the real data is stored in data disk?

I have executed below api GET "/_cat/shards?v" it has shown all the indices in which one of the indices has taken 302 GB.

When I have checked the mounted disk ( mentioned in ) which elastic is being used, it shows only 40GB in the utilization. How come this is possible ? Will data is compressed while storing in the disk ? Any reference doc would be really helpful.

Is this related to How much disk storage required to take backup of index which is having 300GB??