Why is my time machine backup so large after updating from OSX 10.13.6 to 10.15.4? Yesterday, I took my 2015 Macbook Pro running OSX 10.13.6 and backed it up using Time Machine to an external hard drive. I then immediately updated my laptop to 10.15.4. I plugged in my external hard drive to do back it up again using Time Machine, and the subsequent backup was over 70GB(!).  Looking at About This Mac -> Storage, I currently have an 256GB SSD, with 84GB of Photos, 78GB of documents, 18GB of Music, 7GB of Apps, 11GB of System, 16GB of other.  

I can not figure out why this backup was so enormous.  What did Apple change so significantly over the past two operating system iterations? I know they dropped 32bit support for applications from 10.13 -> 10.14.  Could this be why this backup to be so large? 

My cloud backup service (SpiderOak One), also decided that it needed to backup 40GB of stuff. It looks like its doing mostly photos in Photos Library.photoslibrary/originals, but likewise I don't see why it would need to do this.  I have it running constantly and I believe it was working and up to date prior to this backup.  

I believe that these two issues are related. Perhaps Apple changed its file system yet again (like from 10.12 -> 10.13, where it changed  to AFS)?  Maybe it changed how it handles Photos?

Question:
1. Why was my Time Machine and SpiderOak backups so large?  The only thing I did was update my operating system.
First off I can definitely say that the large backup is not caused by the drop of support for 32-bit applications, nor is it due to yet another change of file system from APFS to "something new" (note that Apple used HFS+ for ~20 years before moving to APFS, so file system changes are definitely not done often).

One of the big changes that comes with the update from 10.13 to 10.15 is ofcourse the operating system files themselves. Most of them have changed in some way, so that contributes a lot to the difference. Also Catalina split system and other data over two volumes (while preserving a unified view) so that to a backup system some files could seemingly have moved around.

I can't immediately find a reason for Photos to have made changes to your originals. I would suggest comparing one or two photos in the backups from before and after your upgrade to find the differences - perhaps there has been some kind of meta data change, but I haven't experienced that when upgrading to Catalina.

In general you can use a tool such as BackupLoupe to examine the Time Machine backup made just after upgrading to figure out, which files were changed/moved and made the backup larger.

Another tool that can be used is GrandPerspective, which will visualise the data taking up space in the backup, allowing to you get a high level overview of what taking up the most space.