Why is the index size sometimes less than that reported in Stack usage Statistics?
Logs Sent is calculated from the maximum amount of disk utilitsed by Elasticsearch for a particular day, for the primary data store.
It's important to be aware that when viewing the Elasticsearch index status (using /_cat/indices) this page reports by default in Gibibyte (GiB - https://en.wikipedia.org/wiki/Gibibyte) and Logit reports usage in the more commonly used Gigabite (GB - https://en.wikipedia.org/wiki/Gigabyte), hence a difference when trying to compare the values.
You can always query the elasticsearch index api and return the pri.store.size in bytes (/_cat/indices?h=index,pri.store.size&bytes=b&v), which can then be converted into GB's.
Are the elastalert_status indexes included in the daily usage statistics?
The elastalert_status indexes aren't included in the usage statistics and the cut of time for index size calculation is UTC 00:00. Indexes that are currently in use are normally slightly larger in size than older indexes, this is due to older indexes being compressed.