Muscle doesn’t weigh more than fat. A pound is a pound. However muscle is more dense than fat so a pound of muscle is like a small brick while a pound of fat is like a fluff of cotton balls. To answer your question, to balance the loss of fat, in order to maintain body weight, it’s important to increase lean mass. So, body weight can be maintained, but overall size will decrease. Aggressive resistance training appears to be the most effective way to accomplish this.