Do you have a style sheet attached to the label? Does it add a margin or padding? If so, each time you scale the image you add the margin/padding to it size.

This, for example, does not change the size of the label every two seconds:
Qt Code: 
  1. #include <QtGui>                                                                                                           
  2. #include <QDebug>
  3.  
  4. class MainWindow: public QMainWindow {
  5.     Q_OBJECT
  6. public:
  7.     MainWindow(QWidget *p = 0): QMainWindow(p) {
  8.  
  9.         QWidget *central = new QWidget(this);
  10.         label1 = new QLabel(this);
  11.         image.load("test.png");
  12.         label1->setPixmap(image);
  13.         qDebug() << image.size() << label1->size(); 
  14.         label2 = new QLabel("Label 2", this);
  15.  
  16.         QVBoxLayout *layout = new QVBoxLayout(central);
  17.         layout->addWidget(label1);
  18.         layout->addWidget(label2);
  19.  
  20.         central->setLayout(layout);
  21.         setCentralWidget(central);
  22.  
  23.         connect(&t, SIGNAL(timeout()), SLOT(tick()));
  24.         t.start(2000);
  25.     }
  26. public slots:
  27.     void tick() {
  28.         QPixmap temp = image.scaled(label1->width(), label1->height(), Qt::KeepAspectRatio);
  29.         qDebug() << temp.size() << label1->size(); 
  30.         label1->setPixmap(temp);
  31.     }
  32. private:
  33.     QPixmap image;
  34.     QLabel *label1 ;
  35.     QLabel *label2 ;
  36.     QTimer t;
  37. };
  38.  
  39. int main(int argc, char *argv[])
  40. {
  41.     QApplication app(argc, argv);
  42.  
  43.     MainWindow m;
  44.     m.show();
  45.     return app.exec();
  46. }
  47. #include "main.moc"
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unless you add:
Qt Code: 
  1. qApp->setStyleSheet("QLabel { margin: 5px; }");
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in main().