In this article, the author discusses the download sizes of macOS updates and the discrepancies that can occur on Apple silicon Macs. They explain that macOS updates for Intel Macs are typically around 500 MB, while updates for Apple silicon Macs are never less than 1.8 GB. However, the author discovered that updates for Apple silicon Macs consist of two components. The first component, similar in size to the update for Intel Macs, is downloaded first. Then, Apple silicon Macs download a second component, which is generally about 1.1 GB in size. The author advises paying little attention to the download size given by softwareupdate and instead adding the 1.1 GB overhead to arrive at the true download size. It is important for websites reporting macOS update sizes to specify whether the figures are for Intel or Apple silicon Macs and whether they include the overhead.
https://eclecticlight.co/2023/08/19/macos-updates-for-apple-silicon-macs-are-larger-than-reported/