Overweight animals tend to be poor breeders--BHB just means they like to keep their males trim, without a lot of extra weight--but they still need to be a healthy weight, and well able to withstand the long fast over the winter.

How much an adult male should weigh in order to be 'trim' rather than skinny is going to depend entirely on the individual animal. Some males are quite small, and some are simply not--they can be as long as a female. I would think a 5 foot male to be underweight if it only weighed 1000 grams. I would be more comfortable with an animal that size weighing at least 1500. But males rarely get that long.