Check this place out:
Check out the sizing page and the following sizing for rimmed and belted cases. That may explain a little bit.
As already stated, when you chamber this round, it will stop in the chamber when the belt hits its little indent in the chamber; whereas other rounds stop once the shoulder hits the chamber.
Just use the "feel" method of sizing your cases... Put a fired case back in the chamber and close the bolt (remove the firing pin from the bolt so you can feel the movements better). Remember how that feels. Now, back the die off a bit, size a case, put it back in the chamber. Note how that feels in comparison. Keep sizing till you get a "snug" fit that allows the bolt to close without excess pressure - a little pressure is fine - no pressure at all is bad. You want the case to be almost the exact same size as your chamber. Be sure you have adequate neck tension when you find the best "sweet spot".
Hopefully you have a headspace measuring tool of some sort. If so, take several fired cases and measure them, using the largest one as a reference point. Size the cases and measure in the tool until you get around .001" - .002" shorter.
If you don't have a tool, get one. Some, like the Hornady LnL Case Headspace Gauge kit, which comes with bushings for various calibers, measures off the "datum line" of the shoulder. So does my favorite, the RCBS Case Mic. The best available, that I'm aware of, although I haven't had the priveledge of using one, the Sinclar Should Bump Gauge, measures off the shoulder, not the datum line. Both versions measure to give the same target task - however I feel the Sinclair does it with a bit more precision. I own the Hornady LnL kit, and one RCBS Case Mic, and recommend both.