Generally speaking, if you are using the default revision rules for Exchange Information Store backup, you can expect to use somewhere in the ballpark of 1.8-2.5x the size of the Exchange store in backup usage. If you choose to keep version than 4 versions, your usage will be less; if you keep more versions, your usage will be higher.
Barracuda Intronis Backup
formerly ECHOplatform
formerly ECHOplatform