Why do some joints have very dense smoke and other...

Many things can cause this, but moisture content and resin content can have a big effect. Just smoke hash and you can see what a lot of resin can do when you try to burn it.

Some weed is better dried that others.. and some ppl, roll their joints too tight..(then it doesnt burn/smoke as good) and when ppl blow out dense smoke.. they are not doing it right... Must hold it in your lungs for a gew seconds and then u the smoke u blow out is not as dense ofcourse..but u get HAF! (think u know ;)) Goodluck!

https://thcuniversity.org/the-entourage-effect/

Could be a bit of the environment itself. Low pressure vs high and temps/RH. Then the weed and how its been stored and cured/dried or how its even rolled. I don't think your going to find a lot of useful information here its a very open ended question. But who knows.

So many things could cause this, it's really hard to say with certainty. I certainly know what you mean though. Some strains seem to expand in your lungs when you smoke them lol. I notice it a lot with extracts too so it's certainly possible potency plays a role.