The surprise minor update to macOS 11.2.1 is smaller in size than the 11.2 update: around 2.4 GB for an Intel model, and almost 4.2 GB for an M1 Mac.

The reason for the urgency of this patch is to fix a serious bug in 11.2, in which some MacBook Pro 2016 and 2017 models were unable to charge their battery. It also includes three security fixes, for the recently-reported ten-year-old vulnerability in sudo, and two vulnerabilities in the Intel Graphics Driver. Security release notes are available here.

No bundled Apple apps have changed version number, or have incremented build numbers.

The only other system files which have significant changes in build and/or version number are Apple Intel Graphics kexts.

The version of sudo is incremented from 1.8.31 in 11.2 to 1.9.5p2 in 11.2.1, with matching updates in its policy plugin, file grammar, I/O and audit plugins. You can check sudo’s version numbers using the command
sudo -V
ensuring that the V is a capital.

Although there are no firmware updates for Macs with T2 chips, nor apparently for Apple Silicon models, I suspect there are firmware updates for the affected MacBook Pros. I will confirm this when Apple makes a standalone updater available.

Given the very few files which have been updated, it’s salutory that it requires such large downloads, which must be close to the minimum size for Big Sur updates. Gulp.