I think that depends on their tools and how they evaluate data usage. If the reporting states that the accounts are using very little storage because it's using the same measuring stick that the client does them it's invisible. The question comes up during an audit of the system when the disk usage doesn't match the report. Then again, if this is used by few people it may just look like a margin of error.
It'll more be that the Google docs "live editing" backends are expensive to use disk and memory wise. They store complete version history with each keystroke of a document.
There's a good chance a megabyte of "document" costs Google a gigabyte of internal storage...
They don't store a complete version history. It just uses checkpoints in their timeline of real-time edits and computes the differences when you need them. Those deltas can also be compressed.