Why iOS App Storage Size Mismatches System Settings and How Kuaishou Fixed It

Background

Users notice that the used storage displayed inside many iOS apps does not match the total size shown in the iPhone Settings. The discrepancy can affect user trust and raise concerns about app quality and data security.

Investigation of Differences

Reason 1: App Bundle Size

iOS Settings shows "App Size" and "Documents & Data" separately. Most apps (except QQ) do not include the app bundle size in the "Used" figure shown inside the app.

Reason 2: Binary vs Decimal Units

Manufacturers label storage in decimal (1000) units, while many systems use binary (1024). iOS uses decimal units for its storage display, which leads to a 4.9%–7.4% discrepancy at the MB/GB level.

Reason 3: Measurement Scope

File size (actual bytes) and disk‑occupied size differ because the file system allocates whole blocks (typically 4 KB). Small files therefore occupy more disk space than their actual size, inflating the total.

Reason 4: Path Inclusion

iOS does not count Library/Caches when calculating free space via NSURLVolumeAvailableCapacityForImportantUsageKey . The system excludes this directory from the used‑space total, while some apps count it.

Verification Experiments

Using a jail‑broken iPhone, the author injected files into the sandbox and measured the increase on the "iPhone Storage" page. The increase matched the disk‑occupied size (e.g., 40 MB for 10 000 × 1 KB files), confirming that iOS uses disk‑occupied size for its total.

<code>for i in {1..10000}; do dd if=/dev/zero of=file$i.txt bs=1000 count=1; done</code>

Additional verification used stat.h st_blocks to compute block count (8‑block multiples) and confirmed APFS uses 4 KB logical blocks.

Conclusion

iOS calculates app storage as:

iOS 16 and earlier: App bundle size + sandbox size (excluding Library/Caches)

iOS 17 and later: Sandbox size only (including Library/Caches)

The system uses decimal units and rounds to two decimal places.

Kuaishou’s Alignment

Kuaishou updated its app to:

Include the app bundle size in its own total.

Switch storage unit conversion to decimal.

Calculate total using disk‑occupied size.

Align the path scope with iOS system calculation.

After the changes, the difference between the app’s reported used space and the iPhone Settings total is only 0.43%.

Technical & User Perspective

Accurate storage reporting improves user trust and simplifies storage management, while the technical solution required understanding of APFS block allocation, unit conversion, and system APIs.