ImageView selector with layer-list

Question

The goal of my selector (applied to an ImageView) is to change both color background and image when the state is pressed. The following xml works correctly. The problem is that changing the size of the background also scales up the drawable in the center. How can I keep the the drawable size and the background independent?

<selector xmlns:android="http://schemas.android.com/apk/res/android">

    <item android:state_enabled="true" android:state_pressed="false">
        <layer-list>
            <item>
                <shape android:shape="oval">
                    <size android:width="100dp" android:height="100dp"/>
                    <solid android:color="@android:color/black"/>
                </shape>
            </item>
            <item android:drawable="@drawable/ic_white"/>
        </layer-list>
    </item>

    <item android:state_enabled="true" android:state_pressed="true">
        <layer-list>
            <item android:drawable="@android:color/white" />
            <item android:drawable="@drawable/ic_black" />
        </layer-list>
    </item>

</selector>