diff --git a/internal/integration/crud_prose_test.go b/internal/integration/crud_prose_test.go
index 826dfb4f71..9cbe6e2da6 100644
--- a/internal/integration/crud_prose_test.go
+++ b/internal/integration/crud_prose_test.go
@@ -1017,3 +1017,27 @@ func TestClientBulkWriteProse(t *testing.T) {
 		assert.Equal(mt, num, int(n), "expected %d documents, got: %d", num, n)
 	})
 }
+
+func TestBatchSize(t *testing.T) {
+	mt := mtest.New(t, mtest.NewOptions().ClientType(mtest.Pinned))
+	mt.Setup()
+
+	var hello struct {
+		MaxBsonObjectSize   int
+		MaxMessageSizeBytes int
+	}
+	err := mt.DB.RunCommand(context.Background(), bson.D{{"hello", 1}}).Decode(&hello)
+	require.NoError(mt, err, "Hello error: %v", err)
+
+	var docs []any
+	limit := hello.MaxBsonObjectSize - 30
+	for need := hello.MaxMessageSizeBytes - 350; need > 0; need -= limit {
+		if need >= limit {
+			docs = append(docs, bson.D{{"x", string(make([]byte, limit))}})
+		} else {
+			docs = append(docs, bson.D{{"x", string(make([]byte, need))}})
+		}
+	}
+	_, err = mt.Coll.InsertMany(context.Background(), docs)
+	assert.NoError(mt, err, "InsertMany error: %v", err)
+}
diff --git a/x/mongo/driver/batches.go b/x/mongo/driver/batches.go
index 51a32bc962..ed1b2882bb 100644
--- a/x/mongo/driver/batches.go
+++ b/x/mongo/driver/batches.go
@@ -39,7 +39,7 @@ func (b *Batches) AppendBatchSequence(dst []byte, maxCount, totalSize int) (int,
 	idx, dst = bsoncore.ReserveLength(dst)
 	dst = append(dst, b.Identifier...)
 	dst = append(dst, 0x00)
-	var size int
+	size := len(dst)
 	var n int
 	for i := b.offset; i < len(b.Documents); i++ {
 		if n == maxCount {
@@ -69,7 +69,7 @@ func (b *Batches) AppendBatchArray(dst []byte, maxCount, totalSize int) (int, []
 	}
 	l := len(dst)
 	aidx, dst := bsoncore.AppendArrayElementStart(dst, b.Identifier)
-	var size int
+	size := len(dst)
 	var n int
 	for i := b.offset; i < len(b.Documents); i++ {
 		if n == maxCount {
diff --git a/x/mongo/driver/batches_test.go b/x/mongo/driver/batches_test.go
index 5a028c96dd..5efc57c7ed 100644
--- a/x/mongo/driver/batches_test.go
+++ b/x/mongo/driver/batches_test.go
@@ -36,9 +36,10 @@ func TestAppendBatchSequence(t *testing.T) {
 	batches := newTestBatches(t)
 
 	got := []byte{42}
+	sizeLimit := len(batches.Documents[0]) + len(batches.Documents[1])
 	var n int
 	var err error
-	n, got, err = batches.AppendBatchSequence(got, 2, len(batches.Documents[0]))
+	n, got, err = batches.AppendBatchSequence(got, 2, sizeLimit)
 	assert.NoError(t, err)
 	assert.Equal(t, 1, n)
 
@@ -57,9 +58,10 @@ func TestAppendBatchArray(t *testing.T) {
 	batches := newTestBatches(t)
 
 	got := []byte{42}
+	sizeLimit := len(batches.Documents[0]) + len(batches.Documents[1])
 	var n int
 	var err error
-	n, got, err = batches.AppendBatchArray(got, 2, len(batches.Documents[0]))
+	n, got, err = batches.AppendBatchArray(got, 2, sizeLimit)
 	assert.NoError(t, err)
 	assert.Equal(t, 1, n)