If you think about it, the file storage is in general much more likely to lead to high bandwidth usage than the mail storage.
If you had e.g. a 75MB mailbox and a 75MB quota for file storage, then just filling your file storage allocation ONCE and downloading it again would take 150MB of bandwidth - you could send thousands of normal emails in the same bandwidth!
Since bandwidth = money (from the provider's viewpoint) I think there is some very sharp logic behind the breakdown.
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