Why are there extra pixels around my Android GridView?

I have a GridView in my Android application that has a number of ImageViews in it. The space on my screen is limited and I want the images to take up as much of the available space as they can. Unfortunately, the GridView always leaves 5 pixels of empty screen space around the outside edge of the ImageViews (the space between ImageViews is set with horizontal/vertical spacing and behaves correctly). The empty space acts kind of like a margin around the ImageViews, but I can't get rid of it. Does anyone know what's causing this "border" and how I can get rid of it (or at least make it smaller)? Thanks.

Update: I'm creating the ImageViews by inflating an .xml file in the getView() method of my Adapter class. Here's the xml I'm inflating:

<ImageView
  xmlns:android="http://schemas.android.com/apk/res/android"
  android:background="#FF00FF" />

I defined the GridView in my layout xml file like this:

<GridView
   android:id="@+id/mygrid"
   android:layout_width="fill_parent"
   android:layout_height="wrap_content"
   android:layout_above="@+id/abutton"
   android:layout_marginBottom="8dp"
   android:numColumns="5" android:background="#FF0000"/>

Here's a screen shot of the problem. The red area in my GridView. The purple areas are my ImageViews. The image being displayed is a simple blue rectangle with a transparent center. The image is 45x45 pixels (but is only 30x30 pixels in my app - I'll worry about that later). The red border around the purple is what I am trying to eliminate.