Having different FontSize per platform from XAML

hello I have the following style defined for a label but the font size does not change, is what I am doing possible, if so, what am I doing wrong ?

<Style x:Key="NameLabelStyle" TargetType="Label">
        <Setter Property="FontAttributes" Value="Bold"/>
        <Setter Property="FontSize">
          <Setter.Value>
            <OnPlatform x:TypeArguments="x:String"
                        WinPhone="Large"
                        Android="Medium"
                        iOS="Large"/>
          </Setter.Value>
        </Setter>
      </Style>