Burn,
the larger the displacment the smaller the effect on the idle of the motor the cam has. If you have 400 cubes and installed a cam with 250 degrees at .050, the motor would idle just fine, however if you install that same cam into a 305, it would be absolutely horrible.

I have more duration @ .050 than most guys have advertised duration. I have some 266.272 degrees of duration at .050, I use my stock brakes, a 3500 stall converter, but I have 12:1 compression and 406 cubes.

The larger the cubes the bigger of a vacuum pump your engine is, therefore it can handle a larger cam.