You would need to do in the Xaml for the view containing the MapView. This could be done a number of ways.
My preferred method is to setup a Grid and put the MapView where I want it. But I believe just adding a Margin to the view would suffice.
Grid:
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
...
Title="{Binding Title}">
<Grid WidthRequest="0">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="100" />
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<!-- Could setup Rows to set away from top -->
<Grid x:Name="MainGrid" Grid.Column="1" >
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*" />
</Grid.ColumnDefinitions>
<Grid>
<esri:MapView />
</Grid>
</Grid>
</Grid>
</ContentPage>
Margin:
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms"
xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
...
Title="{Binding Title}">
<-- Margin="Left, Top, Right, Bottom"
<esri:MapView Margin="20,20,0,0"/>
</ContentPage>
Thanks,
-Joe