diff --git a/conformance/internal/conformance/oneof_test.go b/conformance/internal/conformance/oneof_test.go
index af30965..048d9a7 100644
--- a/conformance/internal/conformance/oneof_test.go
+++ b/conformance/internal/conformance/oneof_test.go
@@ -1,16 +1,25 @@
 package conformance
 
 import (
-	"testing"
-
+	"github.com/planetscale/vtprotobuf/types/known/structpb"
 	"github.com/stretchr/testify/require"
 	"google.golang.org/protobuf/proto"
+	upstreamstructpb "google.golang.org/protobuf/types/known/structpb"
+	"testing"
 )
 
-func TestEmptyOneoff(t *testing.T) {
+func TestEmptyOneof(t *testing.T) {
 	// Regression test for https://github.com/planetscale/vtprotobuf/issues/61
-	msg := &TestAllTypesProto3{OneofField: &TestAllTypesProto3_OneofNestedMessage{}}
-	upstream, _ := proto.Marshal(msg)
-	vt, _ := msg.MarshalVTStrict()
-	require.Equal(t, upstream, vt)
+	t.Run("all proto", func(t *testing.T) {
+		msg := &TestAllTypesProto3{OneofField: &TestAllTypesProto3_OneofNestedMessage{}}
+		upstream, _ := proto.Marshal(msg)
+		vt, _ := msg.MarshalVTStrict()
+		require.Equal(t, upstream, vt)
+	})
+	t.Run("structpb", func(t *testing.T) {
+		msg := &structpb.Value{Kind: &upstreamstructpb.Value_StructValue{}}
+		upstream, _ := proto.Marshal((*upstreamstructpb.Value)(msg))
+		vt, _ := msg.MarshalVTStrict()
+		require.Equal(t, upstream, vt)
+	})
 }
diff --git a/features/size/size.go b/features/size/size.go
index 54d856a..23560a2 100644
--- a/features/size/size.go
+++ b/features/size/size.go
@@ -267,9 +267,9 @@ func (p *size) field(oneof bool, field *protogen.Field, sizeName string) {
 	}
 	// Empty protobufs should emit a message or compatibility with Golang protobuf;
 	// See https://github.com/planetscale/vtprotobuf/issues/61
-	// Size is always 3 so just hardcode that here
+	// Size is always keysize + 1 so just hardcode that here
 	if oneof && field.Desc.Kind() == protoreflect.MessageKind && !field.Desc.IsMap() && !field.Desc.IsList() {
-		p.P("} else { n += 3 }")
+		p.P("} else { n += ", strconv.Itoa(key + 1), " }")
 	} else if repeated || nullable {
 		p.P(`}`)
 	}
diff --git a/testproto/pool/pool_with_oneof_vtproto.pb.go b/testproto/pool/pool_with_oneof_vtproto.pb.go
index d781b2a..1c75bbb 100644
--- a/testproto/pool/pool_with_oneof_vtproto.pb.go
+++ b/testproto/pool/pool_with_oneof_vtproto.pb.go
@@ -1139,7 +1139,7 @@ func (m *OneofTest_Test1_) SizeVT() (n int) {
 		l = m.Test1.SizeVT()
 		n += 1 + l + protohelpers.SizeOfVarint(uint64(l))
 	} else {
-		n += 3
+		n += 2
 	}
 	return n
 }
@@ -1153,7 +1153,7 @@ func (m *OneofTest_Test2_) SizeVT() (n int) {
 		l = m.Test2.SizeVT()
 		n += 1 + l + protohelpers.SizeOfVarint(uint64(l))
 	} else {
-		n += 3
+		n += 2
 	}
 	return n
 }
@@ -1167,7 +1167,7 @@ func (m *OneofTest_Test3_) SizeVT() (n int) {
 		l = m.Test3.SizeVT()
 		n += 1 + l + protohelpers.SizeOfVarint(uint64(l))
 	} else {
-		n += 3
+		n += 2
 	}
 	return n
 }
diff --git a/testproto/unsafe/unsafe_vtproto.pb.go b/testproto/unsafe/unsafe_vtproto.pb.go
index 835f096..dd00973 100644
--- a/testproto/unsafe/unsafe_vtproto.pb.go
+++ b/testproto/unsafe/unsafe_vtproto.pb.go
@@ -1572,7 +1572,7 @@ func (m *UnsafeTest_Sub1_) SizeVT() (n int) {
 		l = m.Sub1.SizeVT()
 		n += 1 + l + protohelpers.SizeOfVarint(uint64(l))
 	} else {
-		n += 3
+		n += 2
 	}
 	return n
 }
@@ -1586,7 +1586,7 @@ func (m *UnsafeTest_Sub2_) SizeVT() (n int) {
 		l = m.Sub2.SizeVT()
 		n += 1 + l + protohelpers.SizeOfVarint(uint64(l))
 	} else {
-		n += 3
+		n += 2
 	}
 	return n
 }
@@ -1600,7 +1600,7 @@ func (m *UnsafeTest_Sub3_) SizeVT() (n int) {
 		l = m.Sub3.SizeVT()
 		n += 1 + l + protohelpers.SizeOfVarint(uint64(l))
 	} else {
-		n += 3
+		n += 2
 	}
 	return n
 }
@@ -1614,7 +1614,7 @@ func (m *UnsafeTest_Sub4_) SizeVT() (n int) {
 		l = m.Sub4.SizeVT()
 		n += 1 + l + protohelpers.SizeOfVarint(uint64(l))
 	} else {
-		n += 3
+		n += 2
 	}
 	return n
 }
@@ -1628,7 +1628,7 @@ func (m *UnsafeTest_Sub5_) SizeVT() (n int) {
 		l = m.Sub5.SizeVT()
 		n += 1 + l + protohelpers.SizeOfVarint(uint64(l))
 	} else {
-		n += 3
+		n += 2
 	}
 	return n
 }
diff --git a/types/known/structpb/struct_vtproto.pb.go b/types/known/structpb/struct_vtproto.pb.go
index be8b40e..0699c09 100644
--- a/types/known/structpb/struct_vtproto.pb.go
+++ b/types/known/structpb/struct_vtproto.pb.go
@@ -1003,7 +1003,7 @@ func (m *Value_StructValue) SizeVT() (n int) {
 		l = (*Struct)(m.StructValue).SizeVT()
 		n += 1 + l + protohelpers.SizeOfVarint(uint64(l))
 	} else {
-		n += 3
+		n += 2
 	}
 	return n
 }
@@ -1017,7 +1017,7 @@ func (m *Value_ListValue) SizeVT() (n int) {
 		l = (*ListValue)(m.ListValue).SizeVT()
 		n += 1 + l + protohelpers.SizeOfVarint(uint64(l))
 	} else {
-		n += 3
+		n += 2
 	}
 	return n
 }