Android/xml drawable button, image resizing

Question

I am putting together a simple layout for my android app. This involves using drawable buttons. I put the .png files on the drawable-mdpi directory and called them from the .xml file. Sonce they appeared too big, I scaled them down. What happened was that the image itself indeed became smaller, but the total size of the button stayed the same (the "margins" just became larger). What have I done wrong?

Here's the code:

    <Button
        android:id="@+id/btn1"
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:text="+P"
        android:shape="oval"
        android:onClick="DrawTest2"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:textSize="20sp"
        android:background="#EA0000"
        android:alpha="0.5"/>

    <Button
        android:id="@+id/AddConstraint"
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:onClick="test1"
        android:layout_alignParentBottom="true"
        android:layout_toRightOf="@+id/btn1"
        android:text="+C"
        android:textSize="20sp"
        android:background="#1907DC"
        android:alpha="0.5"/>

    <Button
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:id="@+id/button2"
        android:onClick="DrawTest"
        android:scaleType="centerCrop"
        android:layout_alignParentBottom="true"
        android:layout_toRightOf="@+id/AddConstraint"
        android:background="@drawable/gear3"
        android:scaleX="0.5"
        android:scaleY="0.5"/>

    <Button
        android:layout_height="wrap_content"
        android:layout_width="wrap_content"
        android:onClick="TwoProtocolTest"
        android:layout_alignParentBottom="true"
        android:id="@+id/PlayPauseButton"
        android:layout_toRightOf="@+id/button2"
        android:text="PlayPause"
        android:background="@drawable/playpause"
    />

</RelativeLayout>