genetics + environment / nutrients X love = massive yields.
genetics need to be 97% and better. or your gonna get a massive yielding plant that taste like shit or doesn't get you high to get that kind of yield.
Environment, well see most of us really don't have a perfect set up. including myself. every slight bit your temp, humidity, bugs, Co2, or light exposure is off from perfect you suffer. day after day of it not being perfect will lead to slightly less then perfect yields.
nutrients is a big one. most of us follow a chart to grow. unless you popped hundreds of beans growing that chart out, you won't find 1g/w following a chart. I get damn close to 1g/w with my current set up but thats only if i averaged out the best of the best winning seeds I've ran. I've ran lots and lots of beans through this new house and every now and then one LOVES my set up and has the genes to back it up.
I've had a holy grail kush now thats been pretty small bring in 6-9oz under a 600w. i could of easily fit 2-3 more under the light and probably got the same off them. I'm just now running a handful of the clones off the original mother so i guess we will see soon.
I've also had 9/10 of my plants barely bring in 4 oz of some BS even 2-3 off a few. just bad genetics / genetics that don't like my set up.
gotta keep poppin beans and find the ones that show you the most love. those are YOUR babies!
sog helps a lot too. great light coverage is key. no matter how you achieve it really.