What is /Library/Developer/CoreSimulator/Volumes? (simulator runtime disk images)
Quick answer
Each folder in /Library/Developer/CoreSimulator/Volumes (like iOS_23F77) is a mounted simulator runtime, one per iOS version. They are disk images, so folder tools can show a bigger number than the space they really take: on our Mac du reported 15 to 16 GB per runtime while simctl reported about 8 GB. Remove runtimes with xcrun simctl runtime delete, never by deleting these folders.
What's in there
Since Xcode 15, simulator runtimes are disk images that macOS mounts when needed. /Library/Developer/CoreSimulator on our test Mac had Caches, Cryptex, Images, Profiles and Volumes; each folder in Volumes was a mounted runtime named after its build (iOS_23F77 for iOS 26.5).
Why the size looks wrong
A mounted image shows its expanded contents, so walking it with du or a disk visualizer measures the files inside rather than the image on disk. For the real numbers, ask simctl:
xcrun simctl runtime list -j | grep -E '"version"|"sizeBytes"|"lastUsedAt"'On our Mac: iOS 26.0.1 was 8.0 GB, iOS 26.5 8.5 GB and iOS 27.0 8.1 GB.
Can I delete it?
Delete runtimes you no longer test on, but only through Apple's tool, which unmounts the image first:
xcrun simctl runtime delete <identifier> --dry-run
xcrun simctl runtime delete <identifier>Download one again any time in Xcode › Settings › Components.
Questions
Can I delete the folders in Volumes with Finder?
Don't. They are mounted images owned by the system. Use xcrun simctl runtime delete, which unmounts and removes the image properly.
How much space does a runtime really use?
About 8 GB for recent iOS versions on our test Mac, as reported by xcrun simctl runtime list -j (sizeBytes).