XAML Button sitting in middle of grid cell, despite using END definition?

I am building a Xamarin app with two icons in which I have used the following code:

<?xml version="1.0" encoding="utf-8" ?>
<ContentPage xmlns="http://xamarin.com/schemas/2014/forms" 
         xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml" 
         xmlns:local="clr-namespace:Gas_Sense;assembly=Gas_Sense"
         x:Class="Gas_Sense.HomePage"
         Title="Home Page">
  <ContentPage.Padding>
  <OnPlatform x:TypeArguments="Thickness"
            iOS="7, 40, 7, 10" />
  </ContentPage.Padding>
      <Grid>
              <Grid.RowDefinitions>

              <RowDefinition Height="Auto" />
              <RowDefinition Height="Auto" />
              <RowDefinition Height="120" />
              <RowDefinition Height="Auto" />
              <RowDefinition Height="Auto" />
              <RowDefinition Height="Auto" />
              <RowDefinition Height="*" />
              <RowDefinition Height="30" />


        </Grid.RowDefinitions>

        <Grid.ColumnDefinitions>
            <ColumnDefinition Width="130" />
            <ColumnDefinition Width="*" />

        </Grid.ColumnDefinitions>           

I thought this should put the button in the bottom Right hand corner of my screen. Instead it has done this:

The button sits inside a grid layout on a content page. Why is the button sitting in the middle of its grid? The icon doens't have any whitespace around it.