Whenever there’s a macOS update, I give estimates here of its total download size. This isn’t only important for those who don’t have the benefit of high bandwidth connections, but it also provides an indication of how much has changed in that update. Over the course of different versions from Big Sur onwards it has shown how ‘efficient’ macOS updates are. Currently, a minimal macOS update for an Intel Mac is typically around 500 MB; those for Apple silicon Macs are never less than 1.8 GB. So why do some see much smaller sizes on their M-series Macs?

Thanks to the comment by João Próspero Luis to my article about the 13.5.1 update, I have now realised one cause of substantial discrepancies seen in the sizes of macOS updates for Apple silicon Macs.

Way back in the first updates for M1 Macs in Big Sur, it became clear that updates provided to those models consisted of two distinct components or phases in the update process. The first download was similar in size to that for Intel Macs, and essentially the size given for the update by softwareupdate. In the case of the 13.5.1 update, that was around 500 MB for Intel, and just over 700 MB for Apple silicon Macs. As that part of each update should be similar between different architectures, there’s usually little difference. However, there’s a second component that is only downloaded by Apple silicon Macs, which is generally about 1.1 GB in size, bringing the total size to be downloaded to about 1.8 GB.

If you use the command
softwareupdate -l --include-config-data
or read what’s reported in SilentKnight’s text when there’s a macOS update available, you’re told the size of just that first download, which is the only download for Intel Macs. As that download progresses, it’s decompressed and its ‘software update brain’ calls for the second download, whose size is written only in the log. At that point, the actual download size is posted to the update progress window, giving the true total for the whole update, and it’s that higher figure that I quote in my articles about macOS updates here.

If you run a local Content Caching server, you’ll have noticed a difference in these two phases of downloading the update on Apple silicon Macs: the first can be obtained rapidly from your local server’s cache, but the second 1.1 GB always has to be downloaded direct from Apple’s software update servers and can’t be served from the cache, making it slower to download.

When I first reported this behaviour over two years ago, the two phases appeared to take place the other way around, with the fixed-size direct download occurring first, followed by the cachable component, although that order appears to vary. I returned to explore the phases of updating macOS with the 13.0.1 update last year, where I also explained what has changed since Big Sur.

So if you’re updating an Apple silicon Mac, pay little attention to the download size given by softwareupdate or in SilentKnight, or at least add the fixed 1.1 GB overhead to it to arrive at the download size reported in the update progress window. Websites that report the size of macOS updates also need to make clear whether the figures they give are for Intel or Apple silicon Macs, and whether they include that overhead.