Weighing in on Clydesdales
Offering competition in weight divisions, not just age groups, is an excellent way of increasing the market for your event. It recognizes the athletic ability of larger runners and provides them with an opportunity to win awards (more awards are always better - everybody would like to win something).
If you offer competition by weight, it's best to use the complete Clydesdale weight divisions. RunBig outlines the established divisions. Offering only some weight divisions would be like offering only some age groups.
Consider weight divisions to be equal to age groups and present awards at least three-deep. Require a weigh-in at your race to ensure competitors are the weight they claim, and don't give female divisions cutesy names - it won't attract additional women.