Platform specific styles

So I've got my app working on iOS, Android and UWP. For some reason it looks a lot uglier on iOS than the other two, so I want to add some iOS specific styles in now to make it closer to my desired UI. This largely involves 'beefing' up controls to make them more usable and playing with screen real estate.
The first thing I tried to do is increase the height or font on Entry cells, but this has stopped me at the first hurdle. I'd hoped I could do it app wide as a style:

<Style TargetType="Entry">
                <Setter Property="TextColor" Value="White" />
                <Setter Property="FontSize">
                    <Setter.Value>
                        <OnPlatform iOS="40" Android="25" WinPhone="30" />
                    </Setter.Value>
                </Setter>
            </Style>

But no variations of the above I've tried actually work.
Is there a better way of doing this or is it the best way but the syntax is just wrong?