Disappearing Hard drive space in Big Sur

Welcome and only hope with a careful inventory of what is required and what can be removed, software wise - the situation maybe workable.

Sometimes a restart in Safe Mode ( Shift ) immediately at restart will clear some cache files which will be recreated as needed will further improve the space.

Another method would to reindex the drive Rebuild the Spotlight index on your Mac. Sometimes - the reported Space issue will suddenly increate once this action has been taken.

When Migrating from Old to New - was everything Migrated ? Was the Old running Big Sur also ?

What is “Other” storage on a Mac, and how can I clean it out?

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The final word from Apple on Managing the " Other " Category

• Other: Contains files that don’t fall into the categories listed here. This category primarily includes files and data used by the system, such as log files, caches, VM files, and other runtime system resources. Also included are temporary files, fonts, app support files, and plug-ins. You can't manage the contents of this category. The contents are managed by macOS, and the category varies in size depending on the current state of your Mac.

This is why, if you can afford it, you should think about how much storage you're going to need - and then double it. It's amazing how fast it's eaten up.

Welcome and Thank you too.

Do come back to the ASC Forums if or when additional question may arise