Question Different sizes of Windows Defender Definition Updates ?

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Thank you. I thought that softpedia is safe . I downloaded several apps .
Probably meant to stay out of Softpedia for AV applications & updates. Here's from Wikipedia:

Softpedia has occasionally been used to distribute malware, such as a spyware program named OSX/OpinionSpy that was hidden in screensavers uploaded there along with MacUpdate and VersionTracker in 2010.[8][9] A 2018 report found that Softpedia was relatively safe compared to other download sites, with only 4 of 148 programs tested containing unwanted Internet Explorer add-ons and stating that Softpedia was less vulnerable to man-in-the-middle attacks due to serving more stand-alone installers that do not download executable files.[5] Softpedia displays virtual awards for products free of adware, spyware, and commercial tie-ins.[6]
 
@hakaru_eitara

Agree: only download directly from Microsoft and stay out of softpedia.

And be careful: just because "Microsoft" may appear in the website name/page or URL does not mean that the website is truly Microsoft.

As for update file changes - I would expect the file sizes to change as the update files are edited, corrected, and otherwise updated themselves.

Keep the "big picture" in mind and if in doubt - do not download.

Investigate further. Wait. Especially if there is no immediate problem or issue.

Not uncommon to have some buggy or corrupted update(s) recalled or quietly fixed if and as necessary.
 
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Thanks a lot. Once again, you've made me more aware and cautious—but also a bit fearful. I got several apps including IObit Malware Fighter, Microsoft Update Definitions, and a few others. Sorry for the repetitive question 😔😞
What should I do now?
 
Another vote for uninstalling any and all IOBIT products. The company has a dubious reputation, which you can confirm by doing your own Google searches.

I have to wonder why you would have IOBIT, when you already stated that you have the first-in-class antimalware product: Malwarebytes Premium . . . ?

It is your computer, so it is YOUR decision. You have been asking questions here and getting sound advice. You should follow it. Your computer and data will thank you for that decision.

Have a great day, and stay safe in cyberspace. It IS a dangerous place.

Regards,
Phil