I'm going to take a stab at this one...(micah, if this is wrong please correct me) but i believe it has to do with the depth of the ocean off of the east coast. Long period waves tend to reach deeper into the ocean and therefore the continental shelf will have a more profound impact on the wave height that on our typical short period swells. Friction causes the wave to lose energy and thus lose it's height
If you look at our friends in Cali, they have little to no continental shelf off the coast (blacks for example has a giant canyon) so when they get a long period swell the bottom will not cause the waves to shrink significantly.
Generally speaking, depending how wide and how long the fetch is that generates these waves you can expect the energy to dissipate as it moves away from the storm.
Also worth mentioning is that the sand on the east coast tends to be setup better for short period swells and can't handle the amount of energy a long period swell brings to the coast. Of course you also need to account for how the wave refracts off of inlets, piers, etc...but i think you get the idea.
There is a good basic book out there called Surf Science by Tony Butt. It covers how to forecast waves in easily understandable terms. I read it every time i take a crap...haha.