Within reason, the individual tree weight doesn't matter. A "closed canopy" is when you look up in a forest and can't see any sky, just leaves and branches. That means all the available sunlight is being used by leaves.

So for a given soil and climate, a closed canopy maximizes the CO2 capture in tons per acre/hectare. If you want to produce durable wood products and store the carbon, you generally don't want a lot of little skinny trees. You want the trunks to be big enough to get useful pieces out of it.