diff --git a/swupd/create_manifests_test.go b/swupd/create_manifests_test.go
index a45ecb8..1008dfb 100644
--- a/swupd/create_manifests_test.go
+++ b/swupd/create_manifests_test.go
@@ -478,7 +478,7 @@ func TestContentSizeAcrossVersionsIncludes(t *testing.T) {
 	fullSize := manifests["full"].Header.ContentSize
 
 	checkSize(manifests["test-bundle1"], 4+7+emptySize)
-	checkSize(manifests["test-bundle2"], 5+emptySize)
+	checkSize(manifests["test-bundle2"], 5) // emptySize subtracted out
 
 	// Check that content size does add files from previous updates.
 	ts.copyChroots(10, 20)
@@ -490,7 +490,7 @@ func TestContentSizeAcrossVersionsIncludes(t *testing.T) {
 	manifests = mustParseAllManifests(t, 20, ts.path("www"))
 
 	checkSize(manifests["test-bundle1"], 4+7+10+emptySize)
-	checkSize(manifests["test-bundle2"], 5+8+emptySize)
+	checkSize(manifests["test-bundle2"], 5+8) // emptySize subtracted out
 
 	// os-core should have the same size as before.
 	checkSize(manifests["os-core"], osCoreSize)
diff --git a/swupd/manifest.go b/swupd/manifest.go
index 0327979..283859b 100644
--- a/swupd/manifest.go
+++ b/swupd/manifest.go
@@ -757,6 +757,7 @@ func (m *Manifest) subtractManifestFromManifest(m2 *Manifest) {
 				// this is expensive because we care about order at this point
 				m.Files = append(m.Files[:i], m.Files[i+1:]...)
 				m.Header.FileCount--
+				m.Header.ContentSize -= uint64(f1.Info.Size())
 			}
 
 			// only need to advance the m2.Files index since i now points to the next