Answer: The bottom line is that you will probably not be able to gain enough muscle to fill your old bras.
The body gains muscle based on need. If it undergoes physical stress that it can’t handle easily, it increases strength, size or endurance to compensate. The outward appearance of an increase in size is bigger muscles. It takes more work to gain muscle than it does to keep it, but it does require work. You will also need to take in enough vitamins, minerals, protein and calories to provide energy and substrate for the muscle growth. If you quit resistance training and don’t continue to challenge your body it will no longer have a need for bigger muscles and in an efficient manner, will shed the extra muscle.
Body fat is different. Extra energy that is not used to build new tissue gets stored as fat wherever genetics and hormones dictate. If you eat fewer calories than you need, fat tissue releases fatty acids to make up the difference. Many women lose fat from their breasts when dieting. You can gain some muscle to compensate, but whether you will fill out your old bra depends how much breast size you lost. Losing a cup size will require about an inch of muscle growth on your individual chest muscles in this area to make up the difference. This amount of muscle is rarely achieved by women or men.