Button Positioning with Image Sizes Changes - xamarin

Question

I am playing with Xamarin Forms and I would like to add a Button programmatically. Nothing easier, but I've noted that the results vary greatly if the button has an Image rather than a Text.
Now I have this XAML that defines a three-layered screen (header, content, footer):
<?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:Testing"
x:Class="Testing.MainPage"
Title="MainPage"
NavigationPage.HasNavigationBar="False">
<ContentPage.Content>
<!-- Content -->
<StackLayout x:Name="contentpage" Padding="10,100,10,100">
<!-- Header -->
<StackLayout x:Name="header" Orientation="Horizontal" VerticalOptions="StartAndExpand">
<Button Text="ABCD" VerticalOptions="Center" HorizontalOptions="StartAndExpand"/>
<Button Text="EFGH" VerticalOptions="StartAndExpand" HorizontalOptions="EndAndExpand"/>
</StackLayout>
<!-- /Header -->
<!-- Center -->
<StackLayout x:Name="content" Opacity="1" Orientation="Vertical" VerticalOptions="Start" >
<Button x:Name="test" Text="Fire fire fire" HorizontalOptions="CenterAndExpand" VerticalOptions="Start" />
</StackLayout>
<!-- /Center -->
<!-- Bottom -->
<StackLayout x:Name="buttons" Orientation="Horizontal" VerticalOptions="EndAndExpand">
<Button Text="1" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand" />
<Button Text="2" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand" />
<Button Text="3" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand" />
<Button Text="4" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand" />
<Button Text="5" HorizontalOptions="CenterAndExpand" VerticalOptions="CenterAndExpand" />
</StackLayout>
<!-- /Bottom -->
</StackLayout> <!-- Content -->
</ContentPage.Content>
</ContentPage>
And in the corresponding C# source I add a button to the content:
var custombutton = new Button();
custombutton.HorizontalOptions = LayoutOptions.CenterAndExpand;
custombutton.VerticalOptions = LayoutOptions.Start;
custombutton.Image = "mountain.jpg";
//custombutton.Text = "hello";
content.Children.Add(
custombutton
);
As you can see, I want to change the button's properties by setting an image, but the layout changes dramatically.
As you can see, if I add the button with a text everything's ok (left), but as I change the button's image, the button seems to not obey its vertical options (right), or it adds some weird padding. Of course I would like just to add buttons with texts or images without losing the footer StackLayout, but I don't know what's happening here.
Am I missing something here?
Sizes Update
As suggested, I've tried different layouts, however, I ended up with the same results. This last test is with a Grid, and inside a ScrollView with a StackLayout. Different views and arrangements won't change anything in my project.
What matters is the image size as you can see:
Image sizes here are 1668 × 796 and 355 × 190 pixels.
A fully functional source is here available for download: link to the solution.
Controls Update
Using an Image works flawlessly, regardless of the image size. No clue on why a Button adds such a huge space dependent on the image.