diff --git a/changelog/unreleased/issue-3918 b/changelog/unreleased/issue-3918
new file mode 100644
index 000000000..2cc60bf0d
--- /dev/null
+++ b/changelog/unreleased/issue-3918
@@ -0,0 +1,12 @@
+Bugfix: Correct prune statistics for partially compressed repositories
+
+In a partially compressed repository, one data blob can exist both in an
+uncompressed and a compressed version. This caused the prune statistics to
+become inaccurate and for example report a too high value for the unused size:
+
+> unused size after prune: 16777215.991 TiB
+
+This has been fixed.
+
+https://github.com/restic/restic/issues/3918
+https://github.com/restic/restic/pull/3980
diff --git a/cmd/restic/cmd_prune.go b/cmd/restic/cmd_prune.go
index b2344d3cd..08b847e27 100644
--- a/cmd/restic/cmd_prune.go
+++ b/cmd/restic/cmd_prune.go
@@ -307,7 +307,6 @@ func packInfoFromIndex(ctx context.Context, idx restic.MasterIndex, usedBlobs re
 	// Thus 0 == blob is missing, 1 == blob exists once, >= 2 == duplicates exist
 	idx.Each(ctx, func(blob restic.PackedBlob) {
 		bh := blob.BlobHandle
-		size := uint64(blob.Length)
 		count, ok := usedBlobs[bh]
 		if ok {
 			if count < math.MaxUint8 {
@@ -317,19 +316,7 @@ func packInfoFromIndex(ctx context.Context, idx restic.MasterIndex, usedBlobs re
 				count++
 			}
 
-			if count == 1 {
-				stats.size.used += size
-				stats.blobs.used++
-			} else {
-				// duplicate if counted more than once
-				stats.size.duplicate += size
-				stats.blobs.duplicate++
-			}
-
 			usedBlobs[bh] = count
-		} else {
-			stats.size.unused += size
-			stats.blobs.unused++
 		}
 	})
 
@@ -383,12 +370,22 @@ func packInfoFromIndex(ctx context.Context, idx restic.MasterIndex, usedBlobs re
 			// mark as unused for now, we will later on select one copy
 			ip.unusedSize += size
 			ip.unusedBlobs++
+
+			// count as duplicate, will later on change one copy to be counted as used
+			stats.size.duplicate += size
+			stats.blobs.duplicate++
 		case dupCount == 1: // used blob, not duplicate
 			ip.usedSize += size
 			ip.usedBlobs++
+
+			stats.size.used += size
+			stats.blobs.used++
 		default: // unused blob
 			ip.unusedSize += size
 			ip.unusedBlobs++
+
+			stats.size.unused += size
+			stats.blobs.unused++
 		}
 		if !blob.IsCompressed() {
 			ip.uncompressed = true
@@ -421,6 +418,11 @@ func packInfoFromIndex(ctx context.Context, idx restic.MasterIndex, usedBlobs re
 				ip.usedBlobs++
 				ip.unusedSize -= size
 				ip.unusedBlobs--
+				// same for the global statistics
+				stats.size.used += size
+				stats.blobs.used++
+				stats.size.duplicate -= size
+				stats.blobs.duplicate--
 				// let other occurences remain marked as unused
 				usedBlobs[bh] = 1
 			default:
@@ -686,6 +688,7 @@ func printPruneStats(gopts GlobalOptions, stats pruneStats) error {
 func doPrune(ctx context.Context, opts PruneOptions, gopts GlobalOptions, repo restic.Repository, plan prunePlan) (err error) {
 	if opts.DryRun {
 		if !gopts.JSON && gopts.verbosity >= 2 {
+			Printf("Repeated prune dry-runs can report slightly different amounts of data to keep or repack. This is expected behavior.\n\n")
 			if len(plan.removePacksFirst) > 0 {
 				Printf("Would have removed the following unreferenced packs:\n%v\n\n", plan.removePacksFirst)
 			}