Where does the extra space around my button come from?

I have a simple layout:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
              android:orientation="vertical"
              android:layout_width="fill_parent"
              android:layout_height="fill_parent">
    <LinearLayout android:orientation="horizontal"
                  android:layout_width="fill_parent"
                  android:layout_height="wrap_content"
                  android:padding="5dp"
                  android:id="@+id/acquiring_signal_layout">
        <ProgressBar android:id="@+id/ProgressBar01"
                     android:layout_width="wrap_content"
                     android:layout_height="wrap_content"/>
        <TextView android:id="@+id/acquiring_signal_label"
                  android:text="@string/acquiring_signal"
                  android:layout_width="fill_parent"
                  android:textAppearance="?android:attr/textAppearanceLarge"
                  android:layout_height="wrap_content"
                  android:layout_gravity="center_vertical"
                  android:paddingLeft="5dp"/>
    </LinearLayout>
    <com.google.android.maps.MapView
        xmlns:android="http://schemas.android.com/apk/res/android"
        android:id="@+id/mapview"
        android:clickable="true"
        android:layout_height="wrap_content"
        android:layout_width="fill_parent"
        android:apiKey="..."
        android:layout_weight="1"/>
    <Button
        android:text="@string/send_button"
        android:id="@+id/SendButton"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content">
    </Button>
</LinearLayout>

It works fine, except when it renders, the button at the bottom appe开发者_如何学运维ars to have several (roughly 5, I'd guess) pixels of black margin/space around it. Setting the margin of the button doesn't seem to have any affect. What's going on?

This is from the drawables that are used for the background of the button. There are a couple pixels that are transparent in the button drawable. if you set the drawable to your own, you will notice there is no "empty space".

Have you tried setting a negative margin for the button?