Why is my initial Time Machine backup becoming much larger than the data on the source disk?
Solution 1:
Check to see if you have a Dropbox, OneDrive, or GoogleDrive volume on this machine configured for offline/streaming access.
When Time Machine reaches those directories, it will read the files, and the client (Dropbox, Google Drive, whiatever) will respond by streaming the data, which Time Machine will write to the backup as though they were regular files.
If the on-line storage is large enough, Time Machine will overflow the backup medium while streaming the files.
Exclude such directories.