Yes and no. In cases where there is discrimination between men and women, such as women having to fit into a small weight range and men being able to fit into a large weight range, the courts have ruled that this is not legal. However, it is not illegal to have a requirement to maintain a certain weight as long as it does not end up in discrimination between men and women.
For example, Borgata Casino announced that it will fire members of its Borgata Babe waitstaff if they gain weight. Further, the waitstaff is only given 90 days after pregnancy to get back to their pre-pregnancy weight. The only way that women are allowed a larger uniform, is if they have had a breast augmentation. Some of the waitstaff sued Borgata, but the court ruled that the policy is legal because both male and female waitstaff have weight limits and the waitstaff knew what they were agreeing to when they took the job.