I was confused about this at first, so I'll put in my 2 cents' worth.
Forster's standard "Universal" shellholder jaws are really two jaws in one. Set to the small size, they will work with .223 and similar sized cases, and set to the large size, they work for .308 and similar cases - including 6BR. Yours must be set to the smaller size.
To change to the larger size (perfect for .308/6BR family brass), just take apart the shellholder assembly by removing the two hex-head screws. Don't lose the two springs! Turn the two jaws 180 degrees. Don't flip them over, just turn each one 180 deg. Then re-install the springs, reassemble and mount the unit back onto the press. Takes 90 seconds after you have done it once or twice.
One side of each shellholder jaw will be marked - mine were marked with an "S", indicating that these were the small sides. The other side of each jaw would be, well, --- the large side, oddly enough. Make sure that the jaws are installed so that the "S" mark is face up, not face down.
While you have the assembly apart, clean off any grease and apply a small quantity of light oil. When my new Forster press arrived, the protective shipping grease slightly impeded the motion of the jaws. After cleaning off the heavy grease, the jaws work flawlessly.
Now that I have learned how to use it, I like the adjustable jaws better. It's very quick to install and remove brass, since the jaws open when the ram is down. Then the jaws close automatically (because of the springs) when the ram is raised. There is a theoretical advantage (it probably works, realistically) to allow the brass case to "float" in the horizontal plane, along with the die, to ensure concentric sizing of cases and concentric seating of bullets.
Randy