Taken from https://stackoverflow.com/a/30879360/67808:
You can accomplish this easily using API-specific styles. In your normal values/styles.xml, put something like this:
<style name="floating_action_button"> <item name="android:layout_marginLeft">0dp</item> <item name="android:layout_marginTop">0dp</item> <item name="android:layout_marginRight">8dp</item> <item name="android:layout_marginBottom">0dp</item> </style>
and then under values-v21/styles.xml, use this:
<style name="floating_action_button"> <item name="android:layout_margin">16dp</item> </style>
and apply the style to your FloatingActionButton:
<android.support.design.widget.FloatingActionButton ... style="@style/floating_action_button" ... />

**Update (Oct 2016):**
The correct solution now is to put `app:useCompatPadding="true"` into your FloatingActionButton. This will make the padding consistent between different API versions. However, this still seems to make the default margins off by a little bit, so you may need to adjust those. But at least there's no further need for API-specific styles.
**Previous answer:**
You can accomplish this easily using API-specific styles. In your normal `values/styles.xml`, put something like this:
<style name="floating_action_button"> <item name="android:layout_marginLeft">0dp</item> <item name="android:layout_marginTop">0dp</item> <item name="android:layout_marginRight">8dp</item> <item name="android:layout_marginBottom">0dp</item> </style>
and then under values-v21/styles.xml, use this:
<style name="floating_action_button"> <item name="android:layout_margin">16dp</item> </style>
and apply the style to your FloatingActionButton:
<android.support.design.widget.FloatingActionButton ... style="@style/floating_action_button" ... />
As others have noted, in API <20, the button renders its own shadow, which adds to the overall logical width of the view, whereas in API >=20 it uses the new Elevation parameters which don't contribute to the view width.