Why has my model shrunk ~50% under new version of SU?

I haven’t used SU much since summer, but have kept up with upgrades for the Classic license.
When checking the size of my model for the last 5 years, I noticed it shrunk drastically from 718mb to 374mb:

I don’t see anything missing, all the tags (what used to be called “layers”) are still there. Even the Placemaker surrounding buildings are still there - and they added a lot to that total.
Is this just the new version being more efficient or do I have to go back to my previous version (see files listed above)? I always start a new “version” of my file periodically in case something goes wrong and I don’t detect it right away.
How can I ensure that the model has not “lost” something when SU was upgraded, or for some other reason?

If you are using 2021, please update your profile, 2021 uses a new compression system which reduces the file size by a significant amount. Somewhere around 50%.

Where do I update my profile and how?

Click your face top right, then the little man icon, then preferences, then profile…

What am I looking to update here and how does that relate to the compression issue?
Anyway, if it’s just compression I won’t worry about it, but since I have had instances where parts of my model have just vanished, I am concerned about such a large drop in file size. Do I have to investigate each layer (tag)?

You are looking to update what version of sketchup you are using, we need to know that so we are able to answer questions properly.
Specifically to your question, if you are using 2020 and the file dropped 50% it would suggest a serious problem, but if you were using 2021 then it means it is probably just the new compression algorithm.

I have the latest 2021 and updated my profile accordingly.

Ok, thanks, and is the rest accurate, ios is the mobile app I think.
Basically all the info in the profile can relate to questions and can make a significant difference to what the answer may be. Keeping it up to date helps those answering because we can make one click and see what you have rather than having to ask multiple times what OS, what Version, what Graphic card etc

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OK, I updated my OS too.

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If you want to check the file, use Save a copy as back to SU2020 or 19 you will probably see the file size you expect or very close to it.

Ah, yes, that’s all it was! Whew!

Perhaps SU could issue a message when updating a file saved in the old version to the new 2021 version about the compression? It was a little scary!

When opening a file from an older version, SketchUp warns you that saving it will prevent opening it with older versions.

That is true, but it doesn’t say anything about how the file will be a lot smaller.

The release notes do say this, but I imagine that not everyone reads all of the release notes:

Under the hood: a re-built .SKP

SketchUp 2021 brings to light a significant under-the-hood improvement: a re-build of the .SKP file format. You are likely to notice a significant reduction in file sizes when saving models to the new format, and we’re excited about additional workflow improvements this new file format will unlock going forward.

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I definitely did not read it in the release notes, though to be fair, I only skimmed them.
It would be nice to get a heads-up when saving to the new format from an old format file.
It is nice to have a smaller file to send around.