One thing about Big Sur is that it’s likely to be big. Many of us are hoping that it’s going to be a big improvement on Catalina, and with the introduction of Apple Silicon Macs, its going to support a bigger range of hardware than any version of macOS since 10.5 Leopard, the last to support both PowerPC and Intel models. As I explain here, it’s also likely to be significantly bigger when installed on your Mac, something you might like to start planning for.
Perhaps the most obvious reason for Big Sur systems using more storage space is the fact that many apps will become Universal, containing code which runs natively on both Intel and Apple Silicon Macs. Universal Apps are larger than single-platform equivalents, but much of what makes apps so large isn’t code, it’s resources such as documentation and localised versions of its interface. For example, Silent Knight is 14.7 MB for Intel, and 15.9 MB as a Universal App. The increase in size is more significant when you look at only executable code, though: Ulbow’s 358 KB for Intel grows to 730 KB when Universal. For larger apps that difference will be significant for those who are tight for space.
One interesting question is whether Apple will deliver macOS in Universal form, a single installation which will run on either architecture. The system and its bundled apps contain a lot of code, and together in universal binaries would be significantly larger than the Intel-only version now in beta-testing.
There’s no plan for any ARM-emulation on Intel Macs, which will only be able to run Intel code. But Apple Silicon Macs will have Rosetta 2 to translate Intel binaries to ARM-64 code when Intel-only software is installed. Where this is performed on an Apple Silicon Mac, Rosetta caches its translations, which require storage. Until Apple Silicon Macs are widely available, we won’t know how significant this might be.
System and snapshots
Catalina’s System volume is a little over 11 GB in size, and it’s likely that a single-platform version of Big Sur will be slightly larger than that, perhaps 12 or even 13 GB by the time it has been installed. To understand why you won’t be able to install Big Sur on any disk with much less than 50 GB free you have to remember that the disk has to contain the full download of around 9-10 GB, the new System volume of 12-13 GB, at least a basic Data volume, Recovery, firmware updates as needed, and staging for the installation.
Storage requirements increase dramatically with the first update to be installed. If that replaces most of the original System volume with updated system files, even 50 GB could be insufficient for a fairly modest Big Sur installation. This is due to the way that macOS 11 secures its Signed System Volume, and the fact that the previous system is retained in a snapshot.
For the sake of example, let’s suppose the initial release of Big Sur 11.0 has a System volume of 12 GB, and its first update replaces 8 GB of that. What happens during the update will probably be that the original sealed snapshot containing the system is retained, making it easy to roll back to in the event of problems. The installer then installs the 8 GB update, seals it and makes a new snapshot, which is then mounted and used as the new System volume.
Updating Catalina in this way – without sealing or snapshots – you’d end up with a System volume the same size as before the update, 12 GB. In Big Sur, the new System is still 12 GB, but the snapshot of the previous System takes up another 8 GB, making a total of 20 GB. Add the downloaded updater (around 8 GB maybe), Recovery, firmware updates and staging for the installation, and the total space required during installation is well over 30 GB before you’ve even added your Data volume.
The saved snapshot of the previous system can apparently only be removed by an Apple System updater, so there doesn’t appear to be any easy way of discarding the snapshot made of the previous system until the next update, when we’d expect it to be replaced by the next snapshot, and so on. The ability to remove the snapshot of the previous system is a feature that could perhaps be built into the version of Disk Utility provided in the Recovery system. But Apple doesn’t appear to have done that yet.
Otherwise, the only way a user is likely to be able to remove the snapshot of the previous system and reclaim that space would be to perform a clean install of the new version of Big Sur, which is a lengthy procedure and not the sort of thing you’d want to do a couple of weeks after each update to Big Sur.
The final factor to consider is for those who want to run dual-boot systems on the same disk. Although it has been possible to install multiple versions of previous macOS in the same APFS container, so that they share the same free space, at the moment this isn’t advised for Big Sur. Unless that changes by release, anyone wanting to be able to boot into Big Sur and an earlier version of macOS will have to do so using containers of (relatively) fixed size, which can’t share free space. I suspect that trying to run Big Sur in any container much smaller than 100 GB could become frustrating.
So there are several good reasons that Big Sur is likely to be big, bigger than Catalina. If your current boot disk is getting a bit tight for space, or you’re thinking of a dual-boot system, now’s the time start planning how you’re going to manage the upgrade.