try this: in Layout:

    <?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    android:layout_width="match_parent"
    android:layout_height="match_parent">


    <android.support.design.widget.FloatingActionButton
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:padding="0dp"
        android:scaleType="fitXY"
        android:src="@drawable/map"
        app:backgroundTint="@android:color/transparent"
        app:borderWidth="0dp"
        app:useCompatPadding="true" />


</RelativeLayout>

Now in res/values/dimens.xml:

<resources xmlns:tools="http://schemas.android.com/tools">
    <!-- Default screen margins, per the Android Design guidelines. -->
    <dimen name="fab_margin">16dp</dimen>
    <dimen name="design_fab_image_size" tools:override="true">56dp</dimen>
</resources>

This line is important <dimen name="design_fab_image_size" tools:override="true">56dp</dimen>

output:

you can use fabSize attribute

app:fabSize="normal" 

Try this: android:scaleType="center" and try image size 56dp. It worked for me.