Android: layer list without animation scaling

Question

I'm working first time with animation on Android. And I discovered this fact and I became really angry. I had a template for button with rectangle as background and an image bitmap centered using layer list. OK, now I want to replace bitmap image with an animation. I created animation-list drawable and changed my buttons code:

<layer-list>        
        <item>
            <shape android:shape="rectangle">
                 <gradient
                    android:startColor="#666666"                        
                    android:endColor="#666666"
                    android:angle="90"
                    android:centerY="10%"
                    />
                <corners android:radius="0dp" />            
            </shape>
        </item>
        <item
            android:drawable="@drawable/arrowseq">
        </item>

There is no possibility to avoid this very anoying auto-scaling like for bitmap. Could you please help me or explain WHY?

Lot of thanks!