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The size of a macOS update isn’t just a matter of curiosity for tech enthusiasts, it also holds significance for those with limited bandwidth and serves as an indicator of the extent of changes in the update. A typical macOS update for an Intel Mac is around 500 MB, while for Apple Silicon Macs, it’s never less than 1.8 GB.
However, some users have reported seeing smaller sizes on their M-series Macs, sparking a discussion about the discrepancies in the sizes of macOS updates for Apple Silicon Macs. The answer to this puzzle lies in the structure of the updates themselves. It was noticed with the first updates for M1 Macs in Big Sur that the updates for these models consisted of two distinct components or phases. The first download, similar in size to that for Intel Macs, is essentially the size given for the update by the software update command. For instance, in the case of the 13.5.1 update, this was around 500 MB for Intel, and just over 700 MB for Apple Silicon Macs.
However, there’s a second component that is only downloaded by Apple Silicon Macs, which is generally about 1.1 GB in size. This brings the total size to be downloaded to about 1.8 GB. The size of just this first download is reported when there’s a macOS update available. As the download progresses, it’s decompressed and calls for the second download, whose size is only recorded in the log. The actual download size is then posted to the update progress window, reflecting the true total for the whole update. Interestingly, if you run a local Content Caching server, you’ll notice a difference in these two phases of downloading the update on Apple Silicon Macs. The first can be rapidly obtained from your local server’s cache, but the second 1.1 GB always has to be downloaded directly from Apple’s software update servers and can’t be served from the cache, making it slower to download.
So, when updating an Apple Silicon Mac, the download size given by the software update command or in SilentKnight should be taken with a grain of salt. It’s advisable to add the fixed 1.1 GB overhead to it to arrive at the download size reported in the update progress window. It’s also important for websites reporting the size of macOS updates to clarify whether the figures they provide are for Intel or Apple Silicon Macs, and whether they include that overhead. apple.com
