What an interesting post! While it has never occurred to me before, it suggests that efficiency of storage use must be taken into account, as well as storage quotas.It may suggest that Fastmail's pricing is more competitive than is sometimes claimed.
I believe Fastmail does compress email but, as you say, the discrepancy seems rather large to be explained simply by that. I am wondering if the following might be true:
- there is a problem with garbage collection in the Google File System under some circumstances; or
- Google counts duplicate space (taken up for reliability and performance reasons, while Fastmail definitely does not.
I wonder what ideas others might have.