diff --git a/crates/spacewasm_util/src/file.rs b/crates/spacewasm_util/src/file.rs
index f04c05c..f1d86a0 100644
--- a/crates/spacewasm_util/src/file.rs
+++ b/crates/spacewasm_util/src/file.rs
@@ -47,7 +47,7 @@ impl WasmStream for FileStream {
         } else {
             let m = InnerVec {
                 ptr: buf.as_mut_ptr(),
-                capacity: 4096,
+                capacity: buf.capacity() as u32,
                 len: n as u32,
             };