Depending upon their diet and physical activity, your character's figure will be constantly changing. If you character overeats, they will gain weight; if your character fails to eat enough food, they will begin to lose weight. Depending upon their Consumption level, which resets every 24 hours, walking and sprinting will either cause your character to lose weight or build muscle. A Consumption Level of less than 5 will cause the player to become thinner. To simulate the growth of muscle, the player must have a Consumption Lever that is greater than 5; however, constantly raising your Consumption Level will merely make you fatter, so it may be ideal aim for a Consumption Level of 6. Eating fruits and vegetables will marginally raise your consumption, while junk food and heavier meats will rapidly spike it. Aside from diet and exercise, the use of buffout and its variant other forms will simulate the growth of muscle. 

Craftable at any settlement, the mod also includes a bathroom scale. Upon interaction, the scale will display your current amount of bodyfat and muscle mass. To create the scale, you will need 1 plastic, 4 screws, 2 springs, and 2 pieces of steel. If you do not like your weight, you can always drink the "Default Potion," which will change your weight. If you feel like resetting your Consumption Level, you only need to drink the "Reset Consumption." Upon enabling the mod, both potions are automatically add to your inventory and can be used infinitely. If dropped or sold, both potions can be created at any chemlab, using one piece of bubblegum.

Although the mod does not require anything except the base game, I suggest playing with Caliente's Beautiful Bodies Enhancer -CBBE- at Fallout 4 Nexus - Mods and community. I also recommend that you use my body preset, Full Figured at Fallout 4 Nexus - Mods and community. For the mod to operate perfectly, do not enable it until after your character has left Vault 111; otherwise, the script may not operate correctly.

As a WARNING, versions 1.01, 1.02, and 1.03 of this mod may not be ideal for heavy users of power-armor. Whenever entering a suit of power-armor, portions of my "_FDynamicWeightSystem" quest script may cease. This glitch will make the process of losing weight or building muscle more difficult, as the glitch only affects the walking/sprinting effect. To resolve this issue, the player will need to reinstall the mod after using power-armor.