Why is my *first* time machine backup much larger than my disk space used?
"there's only been a few GB of disk changes since the backup started."

Were you using the VMs? Time Machine is somewhat inefficient in that it will copy an entire file if a single modification is made. So if your VM file is 100 GB in size, and it gets modified, then that's another 100 GB that gets copied.

If the first backup goes on for longer than an hour, then it will included deltas.

Some people prefer to exclude VMs from TM and manage backups for them using some other method. You seem to have c. 300 GB of VM data, so if you're using those regularly, you will add 300 GB to your TM volume every day.