public static class Model {
    public Type type;
}

public static enum Type {
    Big, Medium, Small
}

public void test_enum_ordinal() throws Exception {
    Model model = new Model();
    model.type = Type.Big;

    int serializerFeatures = JSON.DEFAULT_GENERATE_FEATURE & ~SerializerFeature.WriteEnumUsingName.mask;
    String text = JSON.toJSONString(model, serializerFeatures);
    System.out.println(text);
}

JSON.DEFAULT_GENERATE_FEATURE &= ~SerializerFeature.WriteEnumUsingName.mask;