In general, most people do not have the genetics to build big bulky muscle – especially women. If you choose to do lighter weights and do more reps, you will not be improving your metabolism, muscle definition, strength or muscle balance. If anything, you may just maintain your current body shape and condition.
In order for the muscle to grow and change shape, it must be stimulated and overloaded. Do not be afraid to choose a heavier weight and strain a little. Every workout you should strive to do more than the last. That may mean a few more repetitions, a few more pounds to lift or doing a second set.
You must challenge yourself to improve, so if you are going to make the effort, you deserve to get the results – go with the heavier weights – you won’t bulk up.