How is heat output controlled in closed-vent gas fireplace inserts?
It appears that the burner length is the primary difference, but that's not to say that there couldn't be other differences in the valving, etc. You'll probably want to contact the manufacturer if a company representative doesn't appear in this conversation. 


  A unique 18-inch (10,000 Btu) Millivolt Loft burner provides heat and
  ambience for bedroom fireplaces. Check your local codes for applicable
  restrictions.
  
  The 24-inch Multi-sided Loft Burner (36,000 Btu) is ideal for
  Peninsula and See-Through fireboxes. ...


http://docs.homeclick.com/PE-Links/Empire%20Comfort%20Systems/Empire%20Comfort%20Systems-VFRL-18-10-specs.pdf