Up to 4 #14 or smaller fixture wires aren't counted in box fill anyway, so a box that is properly sized for the branch circuit conductors is sufficient. See 314.16 (B)(1) Exception.
410.10 Space for Conductors
Canopies and oulet boxes taken together shall provide adequate space so that luminaire (fixture) conductors and their connecting devices can be properly installed.
The italics are mine.
