By default, Exchange transaction logs are not truncated for a full Instant Recovery backup that does not back up to a storage unit. To truncate logs enable Truncate log after successful Instant Recovery backup in the Exchange properties for the client. Consider carefully before you select this option. Ensure that you have an independent method to retain your snapshots for disaster recovery. Alternatively, you can perform a full Instant Recovery backup to a storage unit.

See Truncating Exchange transaction logs by performing a backup to a storage unit.