How can I shrink the drawable on a button?

I have found a very simple and effective XML solution that doesn't require ImageButton

Make a drawable file for your image as below and use it for android:drawableLeft

<?xml version="1.0" encoding="utf-8"?>
<layer-list xmlns:android="http://schemas.android.com/apk/res/android">

    <item
    android:id="@+id/half_overlay"
    android:drawable="@drawable/myDrawable"
    android:width="40dp"
    android:height="40dp"
    />

</layer-list>

You can set the image size with android:width and android:height properties.

This way you could at least get the same size for different screens.

The drawback is that it is not exactly like fitXY which would scale image width to fit X and scale image height accordingly.

You should use a ImageButton and specify the image in android:src, and set android:scaletype to fitXY

Setting scaled drawable in code

Drawable drawable = getResources().getDrawable(R.drawable.s_vit);
drawable.setBounds(0, 0, (int)(drawable.getIntrinsicWidth()*0.5), 
                         (int)(drawable.getIntrinsicHeight()*0.5));
ScaleDrawable sd = new ScaleDrawable(drawable, 0, scaleWidth, scaleHeight);
Button btn = findViewbyId(R.id.yourbtnID);
btn.setCompoundDrawables(sd.getDrawable(), null, null, null); //set drawableLeft for example