Big Sur Update Needs LOTS of Disk Space
Tony Still
If you haven't yet updated to Big Sur, be aware that it needs a lot of disk space to do so. Unfortunately, if you start installing with too little space, it doesn't spot that and does not fail cleanly so you could end up with nasty problems. Worst case is that you lose some of your own data.Apple says you need 35.5GB free. It seems that you need another 13GB for the installer so call that 49GB. If you're coming from something older than Sierra, the numbers are bigger.
Make sure you check before you start.