I hate to say this, but if you have an ectomorphic frame (small and not really that muscular) you are going to have a very hard time bulking up. Mesomorphics build muscle very easily (I'm more mesomorphic in my lower body), but even then, bulk doesn't come easily.
Genetics play the biggest part in making bulky muscle. Basically, women just don't have the testosterone needed to build big muscles. In fact, lifting heavy actually has the opposite effect on women than it does on men: heavy lifting makes the muscle tighten and compress on a woman. Sure, you get a little muscle building, simply because the muscle is working, but what you see most is the definition of the muscle developing.
Those women you see bulked up for fitness competitions lift weights full time, for hours a day, have a natural propensity to build muscle, and heavily supplement. And even then, when they're not training for competition, they lose some of that bulk, because it's just not healthy to supplement that heavily and work out that stringently for long periods of time.
Lift weights, but don't do it with the goal of bulking up, because it could discourage you if/when it doesn't happen. I mean, I've been lifting weights for years and just can NOT get my arms "big", and it seems like I have to lift and lift and lift to see and feel results. The heavier I lift, the tighter and smaller (but sexier) my arms get, but it takes FOREVER to get them there. Lower body for me, on the other hand, is very easy to build muscle, but even then, it doesn't bulk out. It, again, gets tighter, but very nicely defined. It just doesn't take as long as my upper body does to get results. In fact, I see them in one leg workout.