If you use iCloud Drive or iCloud Photos with Optimise Mac Storage turned on, a storage tool's report of your disk usage can look smaller than what you'd expect from the total size of your library or Drive folder — and the tool isn't wrong. It's accurately measuring something different from what you might assume it's measuring, because of how iCloud represents files that aren't fully downloaded.
What a placeholder or dataless file actually is
When Optimise Mac Storage is enabled, macOS keeps a local copy of files you access often and replaces the rest with a lightweight stand-in — often called a placeholder or, in more technical filesystem terms, a dataless file. The Finder still shows the file's name, icon, and reported size, and opening it triggers a download from iCloud on demand. But until that download happens, the actual bytes are not present on your local disk — only the metadata needed to represent the file and fetch it later.
This is a deliberate and useful feature: it lets a Photos library or iCloud Drive folder that's genuinely larger than your local disk still appear complete and browsable, while only consuming local storage for what you actually use regularly.
Why this makes scan totals genuinely ambiguous
A storage tool walking a folder full of dataless files faces the same kind of ambiguity covered elsewhere in this series with hard links: there's more than one honest number to report, depending on what you're asking.
- Local disk usage — how many bytes are actually occupying space on your drive right now. For a placeholder file, this is close to zero; the file's real content lives in iCloud, not locally.
- Nominal or logical size — the size the file would be if fully downloaded, which is what Finder typically displays in a file's Info panel regardless of whether it's currently local or not.
A tool reporting local disk usage — which is the more useful number if you're trying to understand why your disk specifically is full — will show a folder full of optimised iCloud content as far smaller than its nominal total, because most of that content genuinely isn't consuming local space. This is correct behaviour, not a bug, but it can look like undercounting if you were expecting the tool to reflect your iCloud library's full logical size instead.
Why this trips people up specifically
The confusion tends to surface in a specific scenario: someone knows their Photos library is, say, 400GB in iCloud, but a storage tool reports their local Photos-related folder as a fraction of that. Both numbers are true simultaneously — 400GB exists in iCloud, and a much smaller amount is actually resident on the local disk, because Optimise Mac Storage is doing exactly what it's designed to do.
The practical implication is that a storage tool cannot tell you how much iCloud storage you're using from a local disk scan — that's a separate number, visible in the iCloud section of your Apple ID storage settings, not something a filesystem-level tool has any way to see, since the majority of the data it would need to measure isn't on the local disk at all.
What this means for freeing up local space specifically
If your goal is freeing local disk space rather than reducing your iCloud usage, the interaction with Optimise Mac Storage cuts a specific way: manually deleting a locally-cached copy of a file that's backed by iCloud doesn't delete the file — it just evicts the local copy, turning it back into a placeholder, and the space becomes available again. This is generally safe from a data-loss perspective (the file still exists in iCloud) but means a storage tool's cleanup suggestions around iCloud-backed content should be understood as "free local space by evicting the cache," not "delete this data," which is a meaningfully different and lower-risk action than deleting something that only exists locally.
What a careful storage tool should do here
A tool handling iCloud-backed content responsibly should, at minimum, distinguish placeholder or dataless files from fully-local ones in what it reports, rather than treating a file's nominal size as equivalent to actual disk usage. Some tools go further and flag iCloud-backed items specifically in their results, which helps avoid the confusion of "why does this number look smaller than I expected" before it happens rather than requiring you to already know about this behaviour, as this article has just explained.
iCloud Drive versus iCloud Photos: similar mechanism, separate settings
It's worth being clear that iCloud Drive and iCloud Photos are separate features with separate optimisation settings, even though both rely on the same underlying placeholder mechanism. Optimise Mac Storage for Photos is controlled within the Photos app's own iCloud preferences, while iCloud Drive's equivalent behaviour is controlled through the Files & iCloud settings more broadly. It's possible to have one optimised and not the other, which means a scan can show very different local-versus-nominal-size gaps between your Photos library and your general iCloud Drive folder, depending on how each is independently configured. If a storage tool's numbers for one look surprisingly complete while the other looks surprisingly sparse, checking each feature's setting independently is the right next step rather than assuming a single "iCloud setting" governs both.
What happens when you go over your iCloud storage plan
A related but distinct situation worth mentioning: if your iCloud account is over its storage plan limit, this affects your ability to sync new content to iCloud and can affect Photos and Drive behaviour, but it is a separate constraint from local disk space entirely. A Mac with plenty of free local storage can still be blocked from uploading new photos if the iCloud account itself is full, and conversely a Mac genuinely low on local storage doesn't necessarily mean the iCloud account is anywhere near its own limit. A local storage tool has no visibility into your iCloud plan's remaining capacity at all — that's purely an Apple ID account setting, checked separately from anything a filesystem scan can report.
A quick summary
| Question | Where the answer comes from |
|---|---|
| How much of my disk does this iCloud-heavy folder actually use locally? | A storage tool scanning local disk usage |
| How much am I storing in iCloud overall? | Apple ID → iCloud storage settings, not a local storage tool |
| Is deleting a locally-cached iCloud file safe? | Generally yes — it typically evicts the local cache, not the iCloud original |
Where VolumeLens fits
VolumeLens scans and reports actual local disk usage, flagging sparse and placeholder-style files rather than reporting their full nominal size as if it were all sitting on your drive — because a number that doesn't match what your disk actually holds isn't a useful number. If you're trying to understand local space specifically on a Mac with heavy iCloud usage, that's exactly the distinction the scan is built to preserve. See how it handles your own iCloud-heavy folders by downloading VolumeLens, or read more about the app on the home page.