something wront about QPainter::drawEllipse

my platform is :Qt336,WinXp
In my application,I have to draw an ellipse,like:
void MyClass::Draw(QPainter& painter,QPoint pt)
{
//......
painter.drawEllipse(pt.x,pt.y,3,3);
}

now the problem is:
In my pc,my application is running good!
but if I use the remote login tool with XP from other pc to view this
application,the ellipse I drawed is not good.
the problem is the ellipse size is bigger,not 3.
but if I use the "painter.drawRect(pt.x,pt.y,3,3)"
the rectangle is good in local machine or the remote login machine!

And is somthing wrong with QPainter or GDI+?
thanks!