Hi Tea!
Are light weights wimpy? No. And light is subjective anyway.
The amount of weight that you use for an exercise is really dependent upon your goals. And honestly, everyone's body will adapt to an exercise with a given weight,rep,set or overall volume in a way that may be unique to their genetic build.
But in a general sense, keep this in mind:
If you are interested in gaining strength, chose a weight that will allow you to fatigue or lift to failure within 4 to 6 reps. To gain endurance, you will select a weight that will take more than 12 reps to reach fatigue. To gain the best of both worlds and the hypertrophy (a little more muscle mass) choose a weight that is between 8 to 12 reps to failure/fatigue.
Now, your interest with the actresses who lift light and have great muscle definition, it really has more to do with their body fat percentage. We all have muscles, they are just covered in various layers of body fat.
Is lifting light the right way to go because we are hearing a lot about it by some of the celebs? I think it's one approach of many that may be effective. The beauty of strength training is that for it to be truly effective it needs to be varied every four to six weeks. So, that gives us plenty of room to try things out and prevent plateaus or boredom.
Personally though, I like to lift heavy because I enjoy the feeling of weight pressing into my muscles and fatiguing them. If I want to work on endurance I'll lighten it just a little bit to get within the 12 to 15 rep range. Much more than 15 reps and I really don't enjoy my time "on the bench." My mind starts to wander and I'm no longer able to keep my "mind on the muscle" that's supposed to be engaged and my form will drop. And we all know that this is important to prevent injury from bad form or overuse, and to make the most of the exercise in question. I need to maintain focused concentration and good form otherwise I'm wasting my time and just going through the motions and it will eventually show in my lack of results.