My old Eskie was neutered at the age of 2+ (rescue dog. Don't know exact age, and the surgery was part of the contract), and he stayed just as fat as he was when we got him. As soon as I switched him to lean food & started exercising, he was leaner and trimmer than pre-neutering. He also didn't lose an ounce of his protective abilities.

Similar story with our Chi. He growls just as hard at strangers as he did before.