XML : RelativeLayout takes fullscreen with layout_height="80dp"

I'm trying to create a fragment with height of 80dp, and at Design section it seems to work pretty well, but when I run the app on my device, RelativeLayout somehow takes a fullscreen. I use android:layout_alignParentBottom="true" for the SeekBar, but as far as I know it shouldn't take fullscreen if Layout's height isn't wrap_content. Here is the XML code:

  <?xml version="1.0" encoding="utf-8"?>  <RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"      android:orientation="vertical" android:layout_width="match_parent"      android:layout_height="80dp"      android:background="#bbbbff">        <SeekBar          android:layout_width="fill_parent"          android:layout_height="wrap_content"          android:id="@+id/progressBar"          android:layout_alignParentBottom="true"          android:layout_toLeftOf="@+id/playButton"/>        <Button          android:layout_width="60dp"          android:layout_height="60dp"          android:text="play"          android:layout_alignParentRight="true"          android:id="@+id/playButton"/>  </RelativeLayout>