TimeMachine backs up 25 GB of data every time.

If you have single large files or folders, such as the file used for a virtual machine, or you use an email program with a big database file -- these situations can result in a large file or folder getting backed up even when only a small change was made. For instance, a Parallels Windows virtual machine will back up the entire Windows virtual file (which can be > 50 GB) even if all you do is log in to Windows and then log out. Email setups with very large numbers of emails with attachments and IMAP might result in this also, depending on the email program and provider being used.

In Terminal, you can use the

tmutil compare

command to see what files have changed and were backed up. This can take a long time to run.

I have heard of a program called BackupLoupe that claims to look into backups and tell you what files have been backed up but I have never used it myself.

Also, if you go a week or two between Time Machine backups, it will tend to make larger backups (10 or 20 GB). I am not sure of the reason for that but there may be caches or other internal files/folders that get changed out after such intervals.

Very large Photos Libraries sometimes are "touched" by the MacOS and appear "modified" even though you have not accessed them yourself. This might lead to them being backed up again.

Hello, davidfromohio. Thank you for posting your question to the Apple Support Communities. We understand your situation and how you are noticing that your Time Machine backups seem larger than you would expect. We know that keeping a current backup of your Mac must be important to you and we can certainly help you with this. 

There may be several reasons why you are seeing such a large backup file, or why the backup takes longer than you would expect to complete. To get started on this, we would suggest completing each suggested step in the following Apple Support article: 

If a Time Machine backup takes longer than you expect - Apple Support

One of the steps in the article above is to restart your backup history with Time Machine. This would remove any previous backups that you have saved now, so you would start over with only the files that are stored on your Mac at this time. If you notice any issues when restarting your backup history, the following article may help you: 

If you can't back up or restore your Mac using Time Machine - Apple Support

We hope that the information here helps you get back to using Time Machine as you would like. Please reply back if you have any further questions. 

Cheers!