Lightroom catalogue gets huge due to denoise data

I only recently upgraded to version 14.4 enn now 14.5.1 after completing three major sport events.
What I’ve noticed is that the catalog file size has increased significantly, presumably due to the denoise processing.
In version 14.3, the file size was 320 MB and now in 14.4 it’s 8.02 GB. In the latest version 14.5.1 it's growth to 21 GB.

I had a total of 17,654 shots, of which 2,416 were uploaded to an online photo album on Flickr. Some of the 2,416 photos were processed with denoise. All shots were imported into LR 14.3, and only after finishing the other two events (with their own separate catalogs) did I upgrade to LR 14.4.
I close the catalog at least once a day, always performing an optimize and backup.

I write the xmp files and see that these are also bigger for the denoised raw files. Also the size of the .lrcat-data folder is bigger, but that also seems normal.
Has anyone else noticed this and how to correct?

Ever since the in-file denoise function in version 14.4, Lightroom vastly inflates the xmp sidecar files. These are usually 14 kb large for my 24 mpix photos, but when denoise is applied, they grow to half the file size, some whopping 13 MEGAbyte, see attached.

I also suspect that these operations may also inflate the lrcat catalog file, because that has ballooned from 9 GB to 14.4 GB over the past few months, with me not adding unusual amounts of photos to it. I have about 240k files cataloged and add perhaps around 500-1000 per month. 

This continues in LR version 15.1, which I just tested. 

Windows 11, all latest and greatest updates and drivers. 

Denoise turned off:

Denoise turned on:

"I have noticed thet the catalogue get huge though. It grows very rapidly. Mines has double the last few days of editing."

Look in the catalog folder. There are two files, <catalog>.lrcat (the main database file) and <catalog>.lrcat-data. 

In LR 14.4, denoise, raw details, and super resolution data has been moved from DNG files into the .lrcat-data file.  That's by design, and you will see that file grow by typically 5 to 50 MB for each denoised photo (more for super resolution).

But if you see the .lrcat file (the main database file) grow quickly, that's due to the bug @DClark064 linked to.  As a workaround until it's fixed, turn off Catalog Settings > Metadata > Automatically Write Changes To XMP. 

You may be the victim of a bug in Adobe's software.  You should read the posts by @johnrellis in this thread: https://community.adobe.com/t5/lightroom-classic-ideas/p-bring-back-the-ability-of-enhance-to-create-a-dng-in-the-background-lrc/idi-p/15374596/page/3#comments 

Ellis' posts are always very carefully explained and supported by very thorough testing.

The way to avoid it is to make sure you have Catalog Settings > Metadata > Automatically Write Changes Into XMP unchecked.

I hope that is the solution to your problem.

Why is this?

Because that's how Adobe designed it.

Is there any way to get around this?

No. (Unless you want to downgrade to 14.3)

Like removing some denoise data for older photos and being able to recreate it later if needed?

You can remove the data from the catalog by going into the history panel and selecting the last edit BEFORE the denoise, right click on it and the select "Clear History Above This Step"

Why does all this data gets saved into the backup?

Everything gets backed up. There is no partial backup.