While lights play an important part of getting the best harvest possible, it's not the ONLY part. Choosing the right strain is probably #1 here... if you don't grow a strain that is known for large, dense buds, you can't expect them. But the best thing you can do for your grow is to give it additional potassium and phosphorus during flower... those two nutrients are, first of all, focused pretty much solely on the flowers and making them as large and as dense as possible and, secondly, it's fairly difficult (though not impossible) to overdose on them. If you're growing in soil, you can use bonemeal and Langeinite to top dress (I've never used the bonemeal but when I grew in soil, I would use 1 tablespoon of Langbeinite as a top-dress once a month throughout the grow)... ready made liquid PK boosters are also offered by pretty much every nute line.... start giving the liquid boosters about the 2nd or 3rd week of flower.
Good luck!