A good rule of thumb: if your fingers fit into the brass knucles snuggly like they’re a series of rings, they’re too tight for you.
Brass knuckles should be loose so that your fingers aren’t the part of your body that absorbs the blow, the “knuckle” part of the weapon is only there to hold them in place more easily. They’re not supposed to “grip” your fingers while you punch. In fact, doing so would go against half of the point of brass knuckles, which is to prevent your fingers from breaking when you punch too hard (as often happens when one punches barehanded).
As has been stated, the idea is to a) give your punches a smaller and harder point of contact and b) redirect the impact to your palm/forearm. Used right, they can most definitely kill or cripple someone in one shot.