I'm experiencing the same issue on my iPad, where Google Drive is consuming a considerable amount of storage, despite me not having any files available offline.
This problem is likely due to an issue with the GD application itself, in terms of code instead of storage.
All files are stored only on the cloud (unless saved offline) for mobile devices, and synchronized on PCs and Macs (not on Linux yet, which *****) by default, unless the user customizes the directories/folders that will be synchronized (to conserve valuable storage on your MacBook's drive).
On mobile applications, only an index of the cloud drive's (iCloud, Google Drive, Dropbox, Box.com, etc.) content is being presented. And when a file is selected to be openend, it is then downloaded temporarily to the device's local storage.
The question remains, if no files have been made available offline in the Google Drive app, then why is the index taking suck a large chunk of storage?
Although this question is best suited for the Google Drive's support team, any answered posted here will most certainly be welcomed by me.