Sending large messages to Gmail then deleting them recovers all space
I also find this a fascinating thread! It's hard to imagine what would cause a 3X difference. Binary files are usually sent using base64 encoding, but that only expands the file by a factor of 4/3, and I doubt that FastMail saves attachments as binary files. Maybe Gmail saves several hash tables or other metadata to make it easier to perform searches. But a 3X ratio is huge!
I just sent 5 identical messages with 14.7 MB attachments each from FastMail to Gmail. In FastMail each message size was about 20.25 MB (after base64 encoding), so the total size should be about 101 MB. In Gmail my used space (at the bottom of the page) changed from 1.72 GB to 1.82 GB, so that matches the expected size. I then deleted the 5 messages and permanently deleted them from Trash. My used space went back up to 1.72 GB. So my tests so far don't show anything unusual.
Hmmm ... I think I will try moving messages back and forth using an IMAP email client and see what happens.
Bill
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