diff --git a/src/mp4box/mod.rs b/src/mp4box/mod.rs
index 9ebbe50..1cca042 100644
--- a/src/mp4box/mod.rs
+++ b/src/mp4box/mod.rs
@@ -225,7 +225,7 @@ impl BoxHeader {
 
             Ok(BoxHeader {
                 name: BoxType::from(typ),
-                size: largesize,
+                size: largesize - HEADER_SIZE,
             })
         } else {
             Ok(BoxHeader {