How can I tell what's being backed up by Time Machine? Sometimes, when Time Machine starts a new backup, I see that it'll be several GB of data. I then wonder: What did I save that was so large? Is it perhaps backing up unnecessary data, that I should exclude in the future?

As a Mac developer, I understand that there must be some kind of database that Time Machine maintains in order to know what files need to be backed up. I like to have access to that so that I can stop a large backup process and then check the database for the pending files, and decide whether I want to take further actions.

I am so far unable to find out where the information is stored.

I checked the tmutil tool, but that doesn't seem to offer an option for listing the pending files, either.

I understand that I can use tmutil compare after a backup to see what was backed up, but I like to instead see the pending files in the queue.
The way the backup works is that it does a large base image of EVERYTHING and then each new backup just backs up changes. 

However - If you perform a touch command, or you move files or interact with them in anyway, this can cause it to flag as being a change, and it will back it up again.