Marijuana reduces inflammation, so should help to prevent muscle breakdown. And people tend not to gain weight when they smoke marijuana, supposedly because they are more interested in smoking pot than eating. And stress increases muscle brakdown, so if pot relaxes you, that alone should help to preserve muscle mass, and you grow new muscle when you're sleeping, so if pot makes you sleeping then there is yet another way for pot to help you gain muscle. And less sleep is linked to obesity, so more pot and more sleep should help you to stay lean and strong as long as you get enough exercise and eat right.