The right way to set AdMob banner to match his parent by XML is:
<com.google.ads.AdView
android:id="@+id/ad"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:gravity="center"
ads:adSize="SMART_BANNER"
ads:adUnitId="ca-app-pub_id"
ads:loadAdOnCreate="true"/>
In addition, as mention before, the ability to match parent depends on the advertiser.
If the advertiser supplied the relevant resources, the ad will stretch to match his parent.
For example full size ad on Nexus 4: