Thanks for taking an interest! Compression is something we have been taking a lot of baby steps (and big steps) on in the last year.
Currently the dimension values are not compressed, meaning if you have very large dimension names with high cardinality (ex: full URLs with query params) then you won’t get that great of compression.
What druid version are you using? Since compression has changed notably “recently”, your results are going to vary depending on which version you are using.
If you REALLY want to see what is taking up the space, you can look at the meta.smoosh file in the index.zip blobs from your deep storage (we do this on occasion to see if we can better-optimize some of our data stores)
it will look something like this:
The first row has to do with versioning, but for the other rows:
The first column is the dimension name, and the third and fourth column are the byte bounds per “thingy you can ask about in a query”
The second column is an index to specify which #####.smoosh file you want (usually its just 0 for 00000.smoosh)