Так работает:
    QByteArray data;
    QDataStream out(&data, QIODevice::WriteOnly);
    for(int i = 0; i < 10000;i++)
        out << 1;
    if(!data.isEmpty())
    {
        udpSocket_->writeDatagram(data, hostAddress_, port_);
        qDebug()<<"write message to:"<<hostAddress_<<" "<<port_;
    }
А так нет(в for 0 добавили и всё):
    QByteArray data;
    QDataStream out(&data, QIODevice::WriteOnly);
    for(int i = 0; i < 100000;i++)
        out << 1;
    if(!data.isEmpty())
    {
        udpSocket_->writeDatagram(data, hostAddress_, port_);
        qDebug()<<"write message to:"<<hostAddress_<<" "<<port_;
    }