For weights to have a positive benefit, they need to be heavy, you need to be failing just before or on the final rep allotted. Metabolic resistance work usually has what might be considered "high" rep ranges, usually in the 12-15 area, and usually progressing down to a lower rep range (8-10). None of these rep ranges are considered pure "strength" work (3-5), but that isn't to say you don't get strength gains from them. I would never, ever recommend lifting light, even if you're lifting for 15 reps (why train to be weak?), you should be lifting heavy, and lifting heavier every week. That's something I want to spend a moment on, some of you complain that I make the weights harder every week, but maybe I haven't explained why I do that? The benefit from metabolic weight training comes from the disturbance it creates in your metabolism, but the body being the way it is, it's highly adaptive. By simply making sure we are increasing the weights every workout, is a cheeky little way we can maximise your potential to boost metabolism and grow or retain muscle mass, by shocking up your system and keeping your body constantly guessing. That's also why we never spend more than 4 weeks doing the same thing and why every program is progressively more difficult. It's called progressive intensity and is one of the staple variables a trainer can manipulate to get our fat loss goals. The point is, unless you're in a rehab setting, in which case you wouldn't be lifting weights at all, there's no reason not to lift heavy (as heavy as you can within rep ranges prescribed).
This idea that women should train with light weights for fear of getting "bulky" represents a huge misunderstanding of biology, diet and training. I have no idea where this myth comes from, but training light will just mean you get no results, no bulking sure, but no weight loss or muscle growth, nothing! Getting "bulky" is a result of intensive bodybuilder style training (which I can guarantee none of you are doing) and a caloric surplus (timed protein, carbs, sleep and no alcohol. How many of you can claim this type of lifestyle?). I trained my ass off, thinking only of gaining muscle for years and still didn't gain that much, I love the hubris some people have, the idea that if they touch a weight they'll get "bulky". Heavy weights within caloric restriction will only create density in your muscle, and possibly a slight increase in muscle, if you're eating well you will lose fat to juxtapose the gain in muscle. There are certain hormonal factors that contribute to muscle growth, namely that of testosterone, women produce less than men, that isn't to say women don't produce it and can't gain muscle, because they do, and they can. But the nature is in the expression of that effect, they produce less hence the effect will be less, so ladies, don't worry about lifting weights heavy and frequently, the programs you are on are designed to promote and maintain muscle mass under conditions of physical stress and caloric restriction (as we all know muscle is your metabolism, more muscle= greater metabolism= greater fat loss).