Instead of creating two styles,you could combine them in one:
<Style x:Key="MarginLabel" TargetType="Label">
<Setter Property="FontSize">
<Setter.Value>
<OnIdiom x:TypeArguments="x:Double">
<OnIdiom.Phone>15</OnIdiom.Phone>
<OnIdiom.Tablet>30</OnIdiom.Tablet>
</OnIdiom>
</Setter.Value>
</Setter>
<Setter Property="Margin">
<Setter.Value>
<OnPlatform x:TypeArguments="Thickness">
<On Platform="Android" Value="20,0,0,0" />
</OnPlatform>
</Setter.Value>
</Setter>
</Style>
then use like:
<Label Text="Hello" Style="{StaticResource MarginLabel}"></Label>
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