The wire harness that comes with a 60 amp relay has conductors that are larger, and the relay will last a lot longer when it's operating at a fraction of its rated capacity. The physical size of the 60 amp unit isn't much bigger than the 30. I've stopped buying 30A relays altogether.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!