I'm not sure it makes sense to assume that all launch companies want to scale up and build vastly bigger rockets. SpaceX did it to go after the CRS launches, and now onto larger commercial/military payloads. Their endgame was never lightweight launch class.

Firefly is making it their business plan to go after a lucrative business, and one that is likely to grow dramatically in the coming years. They're competing with the capabilities currently offered to secondary payloads on larger launch vehicles or as primary payloads on e.g. Orbital's solid fuel vehicles. Orbital/ATK is the real competitor here, not SpaceX.