scaling issues

[opcodewriter]

I have only a button:

<Application
        xmlns="http://schemas.wsick.com/fayde"
        xmlns:x="http://schemas.wsick.com/fayde/x"
        xmlns:vms="ViewModels"
        ThemeName="Metro">
    <Application.Resources>
    </Application.Resources>
    <Grid x:Name="LayoutRoot" Background="Green">
        <Grid.DataContext>
            <vms:MainViewModel/>
        </Grid.DataContext>
        <Button Content="AAA" HorizontalAlignment="Left" VerticalAlignment="Top" />
    </Grid>
</Application>

As I resize the web-browser window, the button changes its size too.
If I set a specific width and height, button doesn't resize, so problem is probably with default width and height (auto).
Same issue is with central alignment:
<Button Content="AAA" HorizontalAlignment="Center" VerticalAlignment="Center"/>

Also, it looks like a problem only with button, a check-box or a border do not experience this issue.