diff --git a/internal/lz4block/blocks.go b/internal/lz4block/blocks.go
index a1bfa99e..138083d9 100644
--- a/internal/lz4block/blocks.go
+++ b/internal/lz4block/blocks.go
@@ -8,12 +8,9 @@ const (
 	Block256Kb
 	Block1Mb
 	Block4Mb
+	Block8Mb = 2 * Block4Mb
 )
 
-// In legacy mode all blocks are compressed regardless
-// of the compressed size: use the bound size.
-var Block8Mb = uint32(CompressBlockBound(8 << 20))
-
 var (
 	BlockPool64K  = sync.Pool{New: func() interface{} { return make([]byte, Block64Kb) }}
 	BlockPool256K = sync.Pool{New: func() interface{} { return make([]byte, Block256Kb) }}
diff --git a/internal/lz4stream/block.go b/internal/lz4stream/block.go
index 459086f0..e9646546 100644
--- a/internal/lz4stream/block.go
+++ b/internal/lz4stream/block.go
@@ -224,9 +224,7 @@ func (b *FrameDataBlock) Close(f *Frame) {
 func (b *FrameDataBlock) Compress(f *Frame, src []byte, level lz4block.CompressionLevel) *FrameDataBlock {
 	data := b.data
 	if f.isLegacy() {
-		// In legacy mode, the buffer is sized according to CompressBlockBound,
-		// but only 8Mb is buffered for compression.
-		src = src[:8<<20]
+		data = data[:cap(data)]
 	} else {
 		data = data[:len(src)] // trigger the incompressible flag in CompressBlock
 	}
diff --git a/writer_test.go b/writer_test.go
index 379fb13f..d8afcc19 100644
--- a/writer_test.go
+++ b/writer_test.go
@@ -7,6 +7,7 @@ import (
 	"io"
 	"io/ioutil"
 	"os"
+	"os/exec"
 	"reflect"
 	"strings"
 	"testing"
@@ -285,6 +286,11 @@ func TestWriterLegacy(t *testing.T) {
 			if _, err := io.Copy(out2, zr); err != nil {
 				t.Fatal(err)
 			}
+
+			if len(src) != out2.Len() {
+				t.Fatalf("uncompressed output not correct size. %d != %d", len(src), out2.Len())
+			}
+
 			if !bytes.Equal(out2.Bytes(), src) {
 				t.Fatal("uncompressed compressed output different from source")
 			}
@@ -292,6 +298,57 @@ func TestWriterLegacy(t *testing.T) {
 	}
 }
 
+func TestWriterLegacyCommand(t *testing.T) {
+	_, err := exec.LookPath("lz4")
+	if err != nil {
+		t.Skip("no lz4 binary to test against")
+	}
+
+	goldenFiles := []string{
+		"testdata/vmlinux_LZ4_19377",
+		"testdata/bzImage_lz4_isolated",
+	}
+
+	for _, fname := range goldenFiles {
+		t.Run(fname, func(t *testing.T) {
+			fname := fname
+			t.Parallel()
+
+			src, err := ioutil.ReadFile(fname)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			out := new(bytes.Buffer)
+			zw := lz4.NewWriter(out)
+			if err := zw.Apply(lz4.LegacyOption(true), lz4.CompressionLevelOption(lz4.Fast)); err != nil {
+				t.Fatal(err)
+			}
+			if _, err := io.Copy(zw, bytes.NewReader(src)); err != nil {
+				t.Fatal(err)
+			}
+			if err := zw.Close(); err != nil {
+				t.Fatal(err)
+			}
+
+			// write to filesystem for further checking
+			tmp, err := ioutil.TempFile("", "")
+			if err != nil {
+				t.Fatal(err)
+			}
+			defer os.Remove(tmp.Name())
+			if _, err := tmp.Write(out.Bytes()); err != nil {
+				t.Fatal(err)
+			}
+
+			cmd := exec.Command("lz4", "--test", tmp.Name())
+			if _, err := cmd.Output(); err != nil {
+				t.Fatal(err)
+			}
+		})
+	}
+}
+
 func TestWriterConcurrency(t *testing.T) {
 	const someGiantFile = "testdata/vmlinux_LZ4_19377"