Mainwindow layout change

[Anshuman]

hello friends

Actually i face a problem while going form one Mainwindow to another Mainwindow..
suppose i have mainwindow name Mainwindow1 and another mainwindow name Mainwindow2...now on the pushbutton of Mainwindow1 when i want to go to Mainwindow2 then it works but the layout of the mainwindow changes..its size decreses..

the code i have wrritten in pushbutton of Mainwindow is..

void MainWindow1::on_Clicked_pushButton()
{
        MainWindow2 *m=new Mainwindow();
        m.showFullscreen();
}

void MainWindow1::on_Clicked_pushButton()
{
        MainWindow2 *m=new Mainwindow();
        m.showFullscreen();
}

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but when i perform same task by using diaglog its works fine..so whats the problem..if anyone please suggest..

with regards
Anshuman