Every weight serves a purpose
Lightweight dumbbells are versatile in terms of their benefits, not only can you build muscle mass with these weights but you can build muscular endurance too. If size is your goal you will need to do at least 3 sets of 8 - 12 reps, as a general rule of thumb, and your rest time between sets should be no longer than 60 - 90 seconds. However, if you're training your muscles to perform for an extended period of time you will want to do at least 3 sets of 12 - 20 reps.
It's important not to forget that you must decrease your rest time between sets to about 30 seconds, otherwise you risk decreasing the effectiveness of your endurance workout. If you're a boxer, a cyclist or a runner, such training can improve your performance by leaps and bounds. What is considered lightweight or heavyweight is relative to the lifter. A good test is to see which weight you can lift for 20 reps without difficulty. But generally speaking any dumbbell from 1kg to 8kg is widely considered to be on the lighter end of the scale.
Heavyweight dumbbells are your go-to for building muscle mass. By putting your muscles under immense pressure, you break them down. This sends hormonal signals to your body, causing the muscles to grow back bigger and stronger than before. Once your body has acclimatised to the new weight, in order to keep on growing you must move up a weight. This is something known as progressive overload. By constantly increasing the demand on your body you will reap amazing benefits in terms of size. It's also worth mentioning that unlike lightweight dumbbells, it is not necessary to have lots of reps.
If you're lifting at your limit then 6 - 8 reps should be sufficient. As mentioned previously, what's considered light or heavy is subjective, what is heavy for one person may be light for another. But, the vast majority consider anything from 10kg and above to be relatively heavy. Test yourself and see which weights push you to the edge of your limits. Once you've found a weight that you can only lift for 6 - 8 reps, you will have found your perfect heavyweight dumbbells. Beyond hypertrophy, heavyweight dumbbells are also great for strength training and explosive power training, essentials for athletes and sportsmen.