The BottomNavigationView is not displayed as full width.
It is not supposed to.
According to design guidelines the width of an action can vary between 80dp and 168dp. The two actions you have defined are not sufficient to fill the whole area horizontally.
(As a side note, also according to guidelines the view should contain between three and five actions.)
If you want to fill the space behind the BottomNavigationView, you can set the background color of the view to be the same color as the items background:
<android.support.design.widget.BottomNavigationView
android:background="@color/bottom_view_color"
app:itemBackground="@color/bottom_view_color"
// .... />