There are lots of variables to this question, but these 3 things can vary your outcome dramatically.
- The type of exercise you enjoy
- The food that you eat
- Lifestyle - the time you have to exercise
- Your gender
- Type of Exercise - Firstly it is important that you do an exercise you enjoy otherwise you will not stick to your
lifestyle change - If you enjoy lifting weights then the best thing for you to so is endurance training. Lower your weights so you are able to 10-12 reps before exhausting yourself, this way you will build up the strength without piling on the muscle. I would say though it is always important to do some kind of Cardio (running) as this really burns the calories & keeps you healthy. If weights are not your thing, then Yoga is meant to increase core strength along with relaxation.
- Food - A healthy balanced diet is always advisable for any lifestyle wether you exercise or not. Protein is the product that will build muscle and it is important when you exercise to eat plenty of it as protein will also mend the muscle you worked on, however I would suggest that if you do not want to bulk up then do not over eat protein. It is probably best for you to see a nutritionist about what is the best diet plan for you.
- Gender. Unfortunately this is very true, muscle growth is enhanced by testosterone so ladies will always be less muscular than men.
( I am not a personally trainer so therefore I would recommend you speaking to your local personal trainer who will be able to set you goals and produce an exercise plan tailored to you.)