Whats the load that you wanna carry? Loading capacity of frame would entirely depend on the material used for frame.
Load carrying capacity of the quad would primarily depend on the motor/prop combination and All Up Weight(AUW).
For eg, if you want to lift AUW 2kg including the copter and payload you would have to use motor/prop combination which generates more than 4kg thrust...i.e thrust should be atleast double the payload which will achieve lift.
Now if you reduce the weight to 1.5kg with the same setup, then your motors will generate the same thrust but will have more power at hand for attitude stabilization and flying in general because of reduced weight. Also longer flight times.
If you increase AUW to say 2.5kg with the same setup, then your flight time will decrease and so will maneuverability. Most probably it wont take off at all!
Similarly the maximum prop size thats suitable for the 450 should be 10". So you would have to find a motor which generates maximum thrust with 10" prop efficiently. And IMO you wont find motors which would be able to generate more than 1.2kg thrust with 10" prop. So you would need a larger frame to mount bigger prop/motors/battery to carry the load.
You can read more @
http://www.rcindia.org/multirotors/tutorial-how-to-build-multirotor-from-scratch/