“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.”
Software Update tells an M-series Mac owner the 13.5.1 update is just over 700 MB. The real download is about 1.8 GB, and the extra 1.1 GB can’t come from a local Content Caching server. The true size only shows up in the log and the progress window, after you’ve committed. Anyone on a tethered or metered connection pays for the number Apple left out.