It is not safe to use Botox to reduce muscle mass in your arms or legs. If there is a reason that you want reduced muscle mass, that isn't achieved merely by not exercising, see a nutritionist or general physician who specializes in the issues you are considering.
If there is any person who is suggesting to you that you can use Botox to achieve this goal, I would walk away fast. That sounds dangerous, unethical, and possibly illegal to me. Definitely do NOT consult with anyone about this issue who is not a board-certified plastic surgeon or neurologist.