Well don't, bulking up is actually an extremely hard thing to achieve and requires precision, dedication and specific training programme and testosterone and women have far less of this than men.
So stop worry about getting bulky and concentrate on the benefits such as weight loss, a boosted metabolism and a physique that looks firm, toned and lean!
Plus as muscle tissue is denser than body fat, it occupies less space, meaning that by increasing your muscle mass, you may actually appear leaner, not larger.