diff --git a/proto/table_marshal.go b/proto/table_marshal.go
index db9927a0c7..591c957885 100644
--- a/proto/table_marshal.go
+++ b/proto/table_marshal.go
@@ -2936,7 +2936,7 @@ func Size(pb Message) int {
 func Marshal(pb Message) ([]byte, error) {
 	if m, ok := pb.(newMarshaler); ok {
 		siz := m.XXX_Size()
-		b := make([]byte, 0, siz)
+		b := make([]byte, siz)
 		return m.XXX_Marshal(b, false)
 	}
 	if m, ok := pb.(Marshaler); ok {
@@ -2969,6 +2969,7 @@ func (p *Buffer) Marshal(pb Message) error {
 	if m, ok := pb.(newMarshaler); ok {
 		siz := m.XXX_Size()
 		p.grow(siz) // make sure buf has enough capacity
+		p.buf = p.buf[:siz]
 		p.buf, err = m.XXX_Marshal(p.buf, p.deterministic)
 		return err
 	}
diff --git a/protoc-gen-gogo/generator/generator.go b/protoc-gen-gogo/generator/generator.go
index dd260f142c..5ae25de1b0 100644
--- a/protoc-gen-gogo/generator/generator.go
+++ b/protoc-gen-gogo/generator/generator.go
@@ -2717,7 +2717,7 @@ func (g *Generator) generateCommonMethods(mc *msgCtx) {
 	if gogoproto.IsMarshaler(g.file.FileDescriptorProto, mc.message.DescriptorProto) ||
 		gogoproto.IsUnsafeMarshaler(g.file.FileDescriptorProto, mc.message.DescriptorProto) {
 		if gogoproto.IsStableMarshaler(g.file.FileDescriptorProto, mc.message.DescriptorProto) {
-			g.P("b = b[:cap(b)]")
+			g.P("b = b[:len(b)]")
 			g.P("n, err := m.MarshalToSizedBuffer(b)")
 			g.P("if err != nil {")
 			g.In()
@@ -2731,7 +2731,7 @@ func (g *Generator) generateCommonMethods(mc *msgCtx) {
 			g.P("return xxx_messageInfo_", mc.goName, ".Marshal(b, m, deterministic)")
 			g.P("} else {")
 			g.In()
-			g.P("b = b[:cap(b)]")
+			g.P("b = b[:len(b)]")
 			g.P("n, err := m.MarshalToSizedBuffer(b)")
 			g.P("if err != nil {")
 			g.In()
diff --git a/test/protobuffer/protobuffer.pb.go b/test/protobuffer/protobuffer.pb.go
index ae8df3a673..06c55b7ac8 100644
--- a/test/protobuffer/protobuffer.pb.go
+++ b/test/protobuffer/protobuffer.pb.go
@@ -89,7 +89,7 @@ func (m *PBuffMarshaler) XXX_Marshal(b []byte, deterministic bool) ([]byte, erro
 	if deterministic {
 		return xxx_messageInfo_PBuffMarshaler.Marshal(b, m, deterministic)
 	} else {
-		b = b[:cap(b)]
+		b = b[:len(b)]
 		n, err := m.MarshalToSizedBuffer(b)
 		if err != nil {
 			return nil, err