Google Drive and OneDrive have different storage sizes because they calculate storage usage quite differently.

Here are some differences:

• Google Drive counts files in the trash against your storage quota.

• Google Drive counts versions of files against your storage quota (i.e., if a file is changed twice then it will use double of quota than a file that was changed just once).

• Google Drive does not count files which are in Google Docs format against your storage quota.

• MS Office files in Google Drive take their full size while OneDrive will store these files in MS Office format and compress them (so they will use a fraction of the storage space than on Google Drive).

• OneDrive de-duplicates some files so they take less space in OneDrive. When downloaded, the files are of their correct size. For example, if you have two PDF files in a OneDrive folder and they have exactly the same content, they will use storage on one file.

• OneDrive does not allow upload of files larger than 100MB (files not uploaded because of that are here: https://www.cloudhq.net/system_report?page=synch.)

• OneDrive will not upload temporary files (i.e., files that start with ~$).

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