Android : Why is the image of an ImageView getting stretched?

I have the following layout:ö

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/main_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_gravity="center"
    android:background="@android:color/black"
    android:gravity="center" >

    <ImageView
        android:id="@+id/main_gradient"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:background="@drawable/gradient"
        android:contentDescription="@string/imageview_description"
        android:scaleType="centerInside" />
</RelativeLayout>

The 'gradient' image is a jpg with 400 x 400 pixels. My Screen size is 1920 x 1200. I set the Gravity of my RelativeLayout to be center and my ImageView's height and width are defined to be wrap_content. I tried scaleType="centerInside", fitCenter and so on and centerInParent="true" but nothing works. The ImageView always gets stretched to the screen size.

Why?

How can an ImageView with wrap_content Layout params take the whole screen when the content is 400 x 400. I dont get it.