Resolved Mystery 13 GB of data suddenly appears? (Time machine)

[T Coma]

Maybe the most perplexing situation yet on my 2011 iMac running El Cap. I ran Time Machine around noon today for the daily backup. I used the computer a fair bit today, but only for email, Excel file work and some Squarespace website updates. I manage email well, and I make sure the delete trash is emptied regularly. No big downloads, no unZIPped files, no video/audio. I fired up TM 12 hours later, and it said preparing to backup 13.xx GB. Huh? Where could that have come from? As I mentioned, no new downloads or installations of any kind. The only unusual move I made today was to activate TRIM on my most recently installed 1 TB SSD boot drive. Any ideas about the 13 GB? I scanned "all my files" real quick in Finder, in chronological order, and I only saw a few MB worth of new files today.

 

[T Coma]

I suppose the broader question is, "How do I compare Time Machine backups?" although I was wary of letting the Time Machine backup the questionable data, considering the possibility of malicious code, however slight that may be. Well, I put that on the back burner and TM ran overnight anyway. I now have the updated backup so I just opened the terminal, changed to the directory containing the TM backups, in my case it looked like:

cd "/Volumes/Time Machine/Backups.backupdb/iMac"

and then ran this comparison: tmutil compare [folder name of backup 1] [foldername of backup 2]

So specifically my comparison looked like this:

tmutil compare 2017-07-19-121659 2017-07-20-080556

A quick copy and paste to Excel let me do an easy review of the changes.

 

[MC6800]

How did I not know about tmutil? Learn something every day --thanks!

(now just waiting in suspense about the 13 GB...)