Actually when the pockets are tighter than 4.5" they play a little easier, not harder.
It has to do with the facing angle, the depth of the pocket shelves and the curvature of the pocket shelves, the closer to the center of the pocket you get, the shallower the shelving is....All of those things combined make a diamond pro one of the more difficult tables to play on until you learn how to play on it.
Jaden
The only time I have played on diamonds extensively was at the last Reno open 4 or 5 years back. I wasn't very good back then, but they weren't very difficult at all. Although I am not sure whether or not the were pro ams or not.