It's got to be snug enough to not leak. That's one of the problems with adult incontinence: unlike an infant's body, an adult body has different hip proportions and different curves that make it more difficult for a diaper to have an effective fit. That's why adult diapers always have four or six tapes while infant diapers only need two.
I've gotta go with "snug and tight" here as well. I've tried many diapers over the years and I find that the ones that provide a snug fit typically last longer and make me feel more secure.