Differential backup. Checksumming and deduplication. If the file is from the base distribution of the OS, then chances are it is already in the cloud, so no need to back it up again.
Who is backing up their OS? I thought most people were only backing up their unique data, which by definition wouldn't exist in 'the cloud' already.
Lots of people do - some backup providers charge tiered amounts or 'unlimited' data, and the convenience of getting your OS back exactly the way it was is absolutely worth using an extra 10-20GB of storage.