Dropbox and Google Drive calculate storage usage quite differently.

Here are some differences:

• Google Drive counts files that are in trash against your storage quota.
• Google Drive counts versions of files against your storage quota. If a file is changed twice, it will use double the quota when compared to a file that was changed just once.
• Google Drive does not count files that are in Google Docs format against your storage quota.
• Files stored by third-party systems on Google Drive will be counted against your storage quota.
• Dropbox does not count history or deleted files against your quota.

More about how Google Drive calculates storage quota can be found here: 
https://support.google.com/drive/answer/2375194.