Android : adding android:background="#9AED7B" changing button height

I am new to Android development.

Below is the code I am using.

<RelativeLayout
    xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:background="#6C70EB"
    android:padding="10sp"
    >
    <TextView android:id="@+id/label"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="URL:"
        android:layout_alignBaseline="@+id/entry"
        android:layout_alignParentLeft="true"/>
    <EditText
        android:id="@id/entry"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:layout_toRightOf="@id/label"
        android:layout_alignParentTop="true"
        android:text="http://www..com"

        />
    <Button
        android:id="@+id/ok"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@id/entry"
        android:layout_alignRight="@id/entry"
        android:shadowColor="@android:color/transparent"
        android:text="Ok"
        android:onClick="openFB"

        />
    <Button
        android:id="@+id/cancel"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_toLeftOf="@id/ok"
        android:layout_alignTop="@id/ok"
        android:onClick="resetURL"
        android:shadowColor="@android:color/transparent"
        android:text="Cancel" />
</RelativeLayout>

This gives me output as below.

Now when I add android:background="#9AED7B" in Ok button, it looks like below.

Any idea why height of the Ok button changed after adding background color to it?