The Gigabyte Gamble
Apple heavily encourages users to stay on the latest version of macOS. If you are running an older operating system like Catalina and Apple releases a major upgrade like Big Sur (or newer versions like Monterey or Ventura), your Mac will often attempt to silently download the entire multi-gigabyte installer file in the background. While this is convenient if you intend to upgrade, it is disastrous if you have a strict data cap on your internet connection, if you are tethered to a mobile hotspot, or if you simply do not have the 12+ gigabytes of free hard drive space required to store an installer you have no intention of using right now.
Preventing Automatic Major Upgrades
To protect your bandwidth and hard drive space and stop macOS from automatically downloading massive installer files for new operating systems like Big Sur, you must adjust the Software Update preferences.
Open System Preferences (or System Settings on newer macOS versions) by clicking the Apple logo in the top-left corner. Click on Software Update. Let the panel refresh. Now, look for the Advanced… button (usually located in the bottom right corner of the window) and click it. A dialogue box with several checkboxes will appear. To stop major OS upgrades from pre-downloading, you must uncheck the box labeled Download new updates when available. (You may also want to ensure “Install macOS updates” is unchecked to be safe). Click OK. Your Mac will still check for updates and notify you that Big Sur is available, but it will not initiate the massive download until you explicitly tell it to do so.