Agreeing somewhat with what others have stated, genetics are in my experience the #1 factor in bud density. In perfect conditions under 1000w lights, some strains just always produce light popcorn for me, and some the "rocks." Having done about a dozen different strains, the one that actually produced the densest for me by far was "Master Kush." I believe from whatever bank I used it was the Nirvana version of it. It's actually well known for being very dense, to the point it is actually very high mold risk, which I ran into one time on a couple of my giant tight nugs, and now I top the shit out of it to keep the buds smaller. Still super dense though. It is actually a great indica smoke, and some people do love the bag appeal of super dense buds, where I've had some super dank popcorn type buds and the bag appeal of the loose buds turned them off so I keep master kush in rotation. I got 5 of those seeds by the way, and I think the strain must be pretty stable, as it is kinda older. Normally when I pop half a dozen seeds I get pretty big pheno variances, usually smell and internode size. With this strain they all produced super solid nugs, and the only variable I had was slightly with the spacing and overall height, which really didnt matter because the plant doesnt grow too tall. When I finished these girls I was nervous because the buds only produced at the very top of each bud site with little to no activity on the lower branches, and not overly huge compared to some of my other strains, but in the end they weighed out comparable or more to my other strains due to the heavy density. Like I said before tho, you have to watch out for the mold and keep your humidity down and airflow through those buds. If you google search on this strain I think it's a pretty common issue.