I interpret it differently.
Stars has more fish, therefore the sharks are bigger winners and play more tables to catch them.
The difference between skill level of sharks and the rest of the huge field of players on stars is bigger than that of party poker. at party poker, sharks and fish are closer in skill..
Players winning on party poker don't do so as easily as they could on stars.
I think this is due to the huge popularity of stars over part0 bringing in many more casual players, and the lower rake at stars (especially in sng's).
If anything I'd say stars is softer if you use good table selection.