HTML messages are stored by having a whole .eml blob
in the unfortunately named "mime_headers" column
of the msgs table.

There are user reports that msgs table
takes a lot of space since introduction
of #7486
which allowed us to see that the space is taken by msgs table.
@adbenitez has some logs, can probably add the sizes of the app data, msgs table and number of rows from the log.

This is likely caused by mime_headers column.
We even previously compressed it:
#4129

The next step for figuring it out
is to find out all the cases when mime_headers is used.
Can it be caused by:

1. Just sending a message with large text?
2. Receiving undecryptable messages?
3. Receiving a message from the bot that uses set_html?

If we don't know what causes this, the issue can be closed by adding more information to the log, e.g. with the sum of sizes of mime_headers column and the number of rows that use it and mime_headers size breakdown so we can confirm that mime_headers is the reason.