"Size on disk" means what? OneDrive clarification

In Explorer the Properties menu item reports the size of various Folders as being X but 'Size on disk' as being Y, with Y being often many MB less than X. What precisely does this mean? Is it only related to One Drive and if so why is a One Drive Folder / File shown as having a size on the disk if it is only in the Cloud? The One Drive Folder itself has a size in GB. Does this mean its size in the Cloud, or does it mean many of the files are in fact present on the drive? I did not expect One Drive to have a significant size unless it files have been loaded. As you can see, I am somewhat confused!

In my experience, "size on disk" is larger than "size".

I've always assumed that "size on disk" includes the unused space in all occupied clusters, whereas "size" does not and is therefore smaller.

I know nothing at all about One Drive.

I've learned to not take anything File Explorer says about space and size too seriously.

One drive stores files on the cloud and only leaves pointers on the HD and any files used regularly so you can have 5 gigs of files but only 2 gig on the actual drive on the PC

If you’re asking why are there two listing for file size:

Size: For uncompressed size
Size on disk: For compressed size

For example, a file may be 4 mB in size.
But several factors can influence how much space is needed to store it on the disk:
- is there 4 mB contiguous available on the drive (sectors all next to each other - so the file is all together on the same area of the drive).
- How the disk is laid out (size of the different storage "sections" on the drive)
- any bad sections that may affect the "contiguousness" (I made that word up :) ) of the drive

Applying ntfs compression to a file can make the size on disk smaller than the file size (in some cases much smaller). This is my most extreme example. It is a backup image file of a blank USB drive made using Alex Page's USB Image Tool. The image is of a blank unused usb stick, I restore it when I want to put the usb back to original 'factory' condition. As virtually all of the usb is zeros it compresses very well. Uncompressed it would occupy many GB, compressed it occupies just 10.2MB actual disk space.

I successfully use various strategies and cleaners for freeing up space on my 'small storage' 2-in-1 Tablet PC, but recently it became a more serious issue when 7 GB vanished from the C drive, leaving only 2 GB free (now increased to 5 GB) at the moment when the May (July?) update is due. One (of several) possibility I am examining is the space taken by One Drive on my PC, as I had expected it to be near zero, but in Tree Size it is shown as 16.6 GB. This is why I am trying to get my head around what the size information actually means in One Drive and if 'size on disk' does mean the GB occupied / used on C. If it only refers to a 'shadow', like the placeholders of the past then there is nothing to be gained by removing it. If on the other hand more than 16 GB is really being used up I need to know why and what can be done about it. I use One Drive mainly is a means of rationing the scarce storage available so the 16 GB may mean it is not achieving its purpose.

The C drive has been compressed ---- but Storage Sense continues to insist that around 10 GB is available using compression, Is this an error? Or might I find a way of releasing it if it is not?

I now suspect that the One Drive I have on my PC is not as it should be, it does not seem to be complete. For example in a magazine I have just read that to reduce the size of the Folder by making files only available on demand (yes please!) I should choose Settings, then select 'Save space and download files as you use them'. There is no such option, or any other similar options under Settings. I can open One Drive display in two ways either as a display of folders with their own icons, or as an Explorer page which shows folders with the arrows showing that they are compressed. In neither case can I find a list of options, including the Files on Demand option. It was introduced in an update a while back and my PC has all updates apart from the pending May ie. July one.

I have gone through One Drive All files and where possible switched them to Online Only (they then show the tiny parachute icon). I could not find any batch or multiple mode for this so it took several hours, but now roughly about two thirds are Online Only. I do not know why the other third can not be made Online Only aswell. But I did expect that the amount of storage occupied on C should be reduced significantly. It has not been reduced al all. It remains at Size 16.7 GB and Allocated 14.9 GB, as before.

Why is this? Either my One Drive app is not working correctly, or else I misunderstand how it should work. I have read some articles about One Drive but am no clearer about this or the absence of the required settings in Settings reported in my previous post.

I will stand to be corrected, but the size of the Onedrive folder depends on whether it is the original 'source' of the data or not: if you are looking at the device the data was created on, then the data must occupy space (but not if you have selected 'online only). If you are accessing Onedrive cloud storage from another device, and it hasn't downloaded the file(s), then its folder will be smaller. So it all depends... See here.

To see what is on your PC, try something like Wiztree to analyse it and show the big stuff.